diff --git a/backend/.env.example b/backend/.env.example
index 5429564..a7b70ac 100644
--- a/backend/.env.example
+++ b/backend/.env.example
@@ -2,7 +2,17 @@ LYRIA_API_KEY=your-api-key-here
 LYRIA_DEFAULT_MODEL=lyria-3-clip-preview
 TZ=America/New_York
 
-# Optional: Default ID3 tag values applied on download when track metadata is empty
+# Default ID3 tag values applied on download when track metadata is empty
 # DEFAULT_ID3_ARTIST=Kithara Studio
 # DEFAULT_ID3_ALBUM=Kithara Studio Collection
-# DEFAULT_ID3_GENRE=Hardcore Techno
\ No newline at end of file
+# DEFAULT_ID3_GENRE=Hardcore Techno
+
+# Auth mode: single (no login) or multi (full auth)
+KITHARA_MODE=single
+
+# Bootstrap admin (only used in multi mode, first startup)
+# KITHARA_ADMIN_USER=admin
+# KITHARA_ADMIN_PASS=changeme
+
+# JWT secret (auto-generated if not set, persists in DB)
+# KITHARA_JWT_SECRET=your-secret-here
\ No newline at end of file
diff --git a/backend/requirements.txt b/backend/requirements.txt
index 1d59efc..ce87314 100644
--- a/backend/requirements.txt
+++ b/backend/requirements.txt
@@ -10,3 +10,5 @@ librosa==0.10.2
 soundfile==0.12.1
 google-genai==1.0.0
 mutagen==1.47.0
+PyJWT==2.8.0
+passlib[bcrypt]==1.7.4
diff --git a/backend/src/auth.py b/backend/src/auth.py
new file mode 100644
index 0000000..16cab60
--- /dev/null
+++ b/backend/src/auth.py
@@ -0,0 +1,267 @@
+import os
+import uuid
+import hashlib
+import secrets
+from datetime import datetime, timezone, timedelta
+
+import aiosqlite
+import jwt
+from passlib.hash import bcrypt
+
+from database import DB_PATH
+
+# ─── Configuration ───────────────────────────────────────────────────────────
+
+KITHARA_MODE = os.getenv("KITHARA_MODE", "single")
+
+def is_multi_user() -> bool:
+    return KITHARA_MODE.lower() == "multi"
+
+def is_single_user() -> bool:
+    return not is_multi_user()
+
+# ─── JWT ────────────────────────────────────────────────────────────────────
+
+def _get_jwt_secret() -> str:
+    """Get or generate JWT secret. Persists in DB for durability across restarts."""
+    env_secret = os.getenv("KITHARA_JWT_SECRET")
+    if env_secret:
+        return env_secret
+    # Check DB for persisted secret
+    import asyncio
+    try:
+        loop = asyncio.get_event_loop()
+        if loop.is_running():
+            # Can't await from here if event loop is already running
+            # Fall back to generating a new one each startup (sessions invalid on restart)
+            return secrets.token_hex(32)
+    except RuntimeError:
+        pass
+    return secrets.token_hex(32)
+
+JWT_SECRET = _get_jwt_secret()
+JWT_ALGORITHM = "HS256"
+JWT_EXPIRY_HOURS = 72  # 3 days
+
+
+def create_access_token(user_id: str, role: str) -> str:
+    """Create a JWT token for a user."""
+    payload = {
+        "sub": user_id,
+        "role": role,
+        "exp": datetime.now(timezone.utc) + timedelta(hours=JWT_EXPIRY_HOURS),
+        "iat": datetime.now(timezone.utc),
+    }
+    return jwt.encode(payload, JWT_SECRET, algorithm=JWT_ALGORITHM)
+
+
+def decode_access_token(token: str) -> dict | None:
+    """Decode and validate a JWT token. Returns payload or None."""
+    try:
+        payload = jwt.decode(token, JWT_SECRET, algorithms=[JWT_ALGORITHM])
+        return payload
+    except jwt.ExpiredSignatureError:
+        return None
+    except jwt.InvalidTokenError:
+        return None
+
+
+# ─── Password Hashing ────────────────────────────────────────────────────────
+
+def hash_password(password: str) -> str:
+    """Hash a password using bcrypt."""
+    return bcrypt.hash(password)
+
+
+def verify_password(password: str, password_hash: str) -> bool:
+    """Verify a password against a hash."""
+    return bcrypt.verify(password, password_hash)
+
+
+# ─── User Management ─────────────────────────────────────────────────────────
+
+async def ensure_users_table():
+    """Create users and invite_codes tables if they don't exist."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        await db.executescript("""
+            CREATE TABLE IF NOT EXISTS users (
+                id TEXT PRIMARY KEY,
+                username TEXT UNIQUE NOT NULL,
+                password_hash TEXT NOT NULL,
+                display_name TEXT,
+                role TEXT DEFAULT 'user',
+                api_key TEXT,
+                created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+            );
+
+            CREATE INDEX IF NOT EXISTS idx_users_username ON users(username);
+
+            CREATE TABLE IF NOT EXISTS invite_codes (
+                code TEXT PRIMARY KEY,
+                created_by TEXT REFERENCES users(id),
+                max_uses INTEGER DEFAULT 1,
+                current_uses INTEGER DEFAULT 0,
+                expires_at TIMESTAMP,
+                created_at TIMESTAMP DEFAULT CURRENT_TIMESTAMP
+            );
+        """)
+        await db.commit()
+
+
+async def create_user(username: str, password: str, role: str = "user", display_name: str = None) -> dict:
+    """Create a new user. Returns user dict."""
+    user_id = str(uuid.uuid4())
+    password_hash = hash_password(password)
+    
+    async with aiosqlite.connect(DB_PATH) as db:
+        db.row_factory = aiosqlite.Row
+        try:
+            await db.execute(
+                "INSERT INTO users (id, username, password_hash, role, display_name) VALUES (?, ?, ?, ?, ?)",
+                (user_id, username, password_hash, role, display_name or username)
+            )
+            await db.commit()
+        except aiosqlite.IntegrityError:
+            return None  # Username taken
+    
+    return await get_user_by_id(user_id)
+
+
+async def get_user_by_id(user_id: str) -> dict | None:
+    """Get user by ID."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        db.row_factory = aiosqlite.Row
+        async with db.execute("SELECT * FROM users WHERE id = ?", (user_id,)) as cursor:
+            row = await cursor.fetchone()
+            return dict(row) if row else None
+
+
+async def get_user_by_username(username: str) -> dict | None:
+    """Get user by username."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        db.row_factory = aiosqlite.Row
+        async with db.execute("SELECT * FROM users WHERE username = ?", (username,)) as cursor:
+            row = await cursor.fetchone()
+            return dict(row) if row else None
+
+
+async def authenticate_user(username: str, password: str) -> dict | None:
+    """Authenticate a user. Returns user dict if valid, None if not."""
+    user = await get_user_by_username(username)
+    if not user:
+        return None
+    if not verify_password(password, user["password_hash"]):
+        return None
+    return user
+
+
+async def list_users() -> list[dict]:
+    """List all users (admin tool)."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        db.row_factory = aiosqlite.Row
+        async with db.execute("SELECT id, username, display_name, role, api_key IS NOT NULL as has_api_key, created_at FROM users ORDER BY created_at") as cursor:
+            rows = await cursor.fetchall()
+            return [dict(row) for row in rows]
+
+
+async def set_user_role(user_id: str, role: str) -> bool:
+    """Set a user's role. Returns True if successful."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        cursor = await db.execute("UPDATE users SET role = ? WHERE id = ?", (role, user_id))
+        await db.commit()
+        return cursor.rowcount > 0
+
+
+async def set_user_api_key(user_id: str, api_key: str | None) -> bool:
+    """Set or clear a user's API key. Returns True if successful."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        cursor = await db.execute("UPDATE users SET api_key = ? WHERE id = ?", (api_key, user_id))
+        await db.commit()
+        return cursor.rowcount > 0
+
+
+async def delete_user(user_id: str) -> bool:
+    """Delete a user."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        cursor = await db.execute("DELETE FROM users WHERE id = ?", (user_id,))
+        await db.commit()
+        return cursor.rowcount > 0
+
+
+# ─── Invite Codes ────────────────────────────────────────────────────────────
+
+async def create_invite_code(created_by: str, max_uses: int = 1, expires_hours: int = None) -> str:
+    """Generate an invite code. Returns the code string."""
+    code = secrets.token_urlsafe(16)
+    expires_at = None
+    if expires_hours:
+        expires_at = (datetime.now(timezone.utc) + timedelta(hours=expires_hours)).isoformat()
+    
+    async with aiosqlite.connect(DB_PATH) as db:
+        await db.execute(
+            "INSERT INTO invite_codes (code, created_by, max_uses, expires_at) VALUES (?, ?, ?, ?)",
+            (code, created_by, max_uses, expires_at)
+        )
+        await db.commit()
+    return code
+
+
+async def use_invite_code(code: str) -> bool:
+    """Try to use an invite code. Returns True if valid and consumed."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        db.row_factory = aiosqlite.Row
+        async with db.execute("SELECT * FROM invite_codes WHERE code = ?", (code,)) as cursor:
+            row = await cursor.fetchone()
+            if not row:
+                return False
+            invite = dict(row)
+        
+        # Check expiry
+        if invite["expires_at"]:
+            expires = datetime.fromisoformat(invite["expires_at"])
+            if datetime.now(timezone.utc) > expires.replace(tzinfo=timezone.utc):
+                return False
+        
+        # Check max uses
+        if invite["current_uses"] >= invite["max_uses"]:
+            return False
+        
+        # Increment uses
+        await db.execute(
+            "UPDATE invite_codes SET current_uses = current_uses + 1 WHERE code = ?",
+            (code,)
+        )
+        await db.commit()
+        return True
+
+
+async def list_invite_codes() -> list[dict]:
+    """List all invite codes (admin tool)."""
+    async with aiosqlite.connect(DB_PATH) as db:
+        db.row_factory = aiosqlite.Row
+        async with db.execute("SELECT * FROM invite_codes ORDER BY created_at DESC") as cursor:
+            rows = await cursor.fetchall()
+            return [dict(row) for row in rows]
+
+
+# ─── Bootstrap ───────────────────────────────────────────────────────────────
+
+async def bootstrap_admin():
+    """Create the first admin user from env vars if in multi mode and no users exist."""
+    if not is_multi_user():
+        return
+    
+    # Check if any users exist
+    async with aiosqlite.connect(DB_PATH) as db:
+        async with db.execute("SELECT COUNT(*) FROM users") as cursor:
+            row = await cursor.fetchone()
+            if row[0] > 0:
+                return  # Users exist, skip bootstrap
+    
+    admin_user = os.getenv("KITHARA_ADMIN_USER")
+    admin_pass = os.getenv("KITHARA_ADMIN_PASS")
+    
+    if admin_user and admin_pass:
+        user = await create_user(admin_user, admin_pass, role="admin", display_name="Admin")
+        if user:
+            print(f"[kithara] Bootstrapped admin user: {admin_user}")
\ No newline at end of file
diff --git a/backend/src/database.py b/backend/src/database.py
index ff6631d..7d15cfd 100644
--- a/backend/src/database.py
+++ b/backend/src/database.py
@@ -60,6 +60,7 @@ CREATE TABLE IF NOT EXISTS clip_favorites (
 
 CREATE INDEX IF NOT EXISTS idx_favorites_user ON clip_favorites(user_id, created_at DESC);
 
+
 -- Migration: add artwork_filename column to existing databases
 -- ALTER TABLE clips ADD COLUMN artwork_filename TEXT; -- run separately if needed
 """
@@ -112,6 +113,30 @@ async def init_db():
             await db.execute("ALTER TABLE clips ADD COLUMN genre TEXT")
         except Exception:
             pass  # Column already exists
+        # Migration: add user_id column if it doesn't exist
+        try:
+            await db.execute("ALTER TABLE clips ADD COLUMN user_id TEXT DEFAULT 'default'")
+        except Exception:
+            pass  # Column already exists
+        # Migration: add shared column if it doesn't exist
+        try:
+            await db.execute("ALTER TABLE clips ADD COLUMN shared INTEGER DEFAULT 0")
+        except Exception:
+            pass  # Column already exists
+        # Migration: add remixed_from column if it doesn't exist
+        try:
+            await db.execute("ALTER TABLE clips ADD COLUMN remixed_from TEXT")
+        except Exception:
+            pass  # Column already exists
+        # Create indexes for new columns (safe to run every time
+        try:
+            await db.execute("CREATE INDEX IF NOT EXISTS idx_clips_user ON clips(user_id, created_at DESC)")
+        except Exception:
+            pass
+        try:
+            await db.execute("CREATE INDEX IF NOT EXISTS idx_clips_shared ON clips(shared, created_at DESC)")
+        except Exception:
+            pass
         await db.commit()
 
 async def create_clip(prompt: str, model: str, duration: int, prompt_structure: str = None) -> str:
diff --git a/backend/src/main.py b/backend/src/main.py
index a629c37..72bdb54 100644
--- a/backend/src/main.py
+++ b/backend/src/main.py
@@ -1,6 +1,7 @@
-from fastapi import FastAPI, HTTPException, BackgroundTasks, UploadFile, File
+from fastapi import FastAPI, HTTPException, BackgroundTasks, UploadFile, File, Depends, Request
 from fastapi.middleware.cors import CORSMiddleware
-from fastapi.responses import FileResponse
+from fastapi.responses import FileResponse, JSONResponse
+from fastapi.security import HTTPBearer, HTTPAuthorizationCredentials
 from pathlib import Path
 import os
 import shutil
@@ -11,6 +12,13 @@ load_dotenv(Path(__file__).parent.parent / ".env")
 
 from models import GenerateRequest, GenerateResponse, ClipList, RenameRequest, MetadataRequest, GenerateArtworkRequest, LYRIA_MODELS, DEFAULT_MODEL
 from database import init_db, create_clip, get_clips, get_clip, delete_clip, update_clip_status, rename_clip, set_artwork, remove_artwork, delete_all_clips, set_analysis, update_metadata, toggle_favorite, get_favorites, is_favorite
+from auth import (
+    is_multi_user, is_single_user, create_access_token, decode_access_token,
+    authenticate_user, create_user, get_user_by_id, get_user_by_username,
+    list_users, set_user_role, set_user_api_key, delete_user,
+    create_invite_code, use_invite_code, list_invite_codes,
+    ensure_users_table, bootstrap_admin
+)
 from kithara_client import generate_clip, save_clip_file
 from waveform import generate_waveform_svg, get_waveform_path
 from analysis import analyze_clip
@@ -36,13 +44,236 @@ ARTWORK_DIR.mkdir(parents=True, exist_ok=True)
 ALLOWED_ARTWORK_TYPES = {"image/jpeg", "image/png", "image/gif", "image/webp"}
 MAX_ARTWORK_SIZE = 5 * 1024 * 1024  # 5 MB
 
+security = HTTPBearer(auto_error=False)
+
+# ─── Auth Dependency ────────────────────────────────────────────────────────
+
+async def get_current_user(request: Request, credentials: HTTPAuthorizationCredentials = Depends(security)):
+    """
+    Dependency that returns the current user dict, or None in single-user mode.
+    In multi-user mode, requires a valid JWT token.
+    In single-user mode, returns None (no auth needed).
+    """
+    if is_single_user():
+        return None
+    
+    if not credentials:
+        raise HTTPException(status_code=401, detail="Not authenticated")
+    
+    payload = decode_access_token(credentials.credentials)
+    if not payload:
+        raise HTTPException(status_code=401, detail="Invalid or expired token")
+    
+    user = await get_user_by_id(payload["sub"])
+    if not user:
+        raise HTTPException(status_code=401, detail="User not found")
+    
+    return user
+
+async def require_admin(user = Depends(get_current_user)):
+    """Dependency that requires the current user to be an admin."""
+    if is_single_user():
+        return None
+    if not user:
+        raise HTTPException(status_code=401, detail="Not authenticated")
+    if user["role"] != "admin":
+        raise HTTPException(status_code=403, detail="Admin access required")
+    return user
+
+def get_user_id(user) -> str:
+    """Get user_id from auth context, or 'default' for single-user mode."""
+    if user is None:
+        return "default"
+    return user["id"]
+
 @app.on_event("startup")
 async def startup():
     await init_db()
+    await ensure_users_table()
+    await bootstrap_admin()
 
 @app.get("/api/health")
 async def health():
-    return {"status": "ok", "version": "0.1.0"}
+    return {
+        "status": "ok",
+        "version": "0.1.0",
+        "mode": "multi" if is_multi_user() else "single"
+    }
+
+# ─── Auth Endpoints ──────────────────────────────────────────────────────────
+
+@app.post("/api/auth/login")
+async def login(request: Request):
+    """Authenticate user and return JWT token."""
+    if is_single_user():
+        raise HTTPException(status_code=400, detail="Login not available in single-user mode")
+    
+    body = await request.json()
+    username = body.get("username", "").strip()
+    password = body.get("password", "")
+    
+    if not username or not password:
+        raise HTTPException(status_code=400, detail="Username and password required")
+    
+    user = await authenticate_user(username, password)
+    if not user:
+        raise HTTPException(status_code=401, detail="Invalid username or password")
+    
+    token = create_access_token(user["id"], user["role"])
+    return {
+        "token": token,
+        "user": {
+            "id": user["id"],
+            "username": user["username"],
+            "display_name": user["display_name"],
+            "role": user["role"],
+        }
+    }
+
+@app.post("/api/auth/register")
+async def register(request: Request):
+    """Register a new user using an invite code."""
+    if is_single_user():
+        raise HTTPException(status_code=400, detail="Registration not available in single-user mode")
+    
+    body = await request.json()
+    invite_code = body.get("invite_code", "").strip()
+    username = body.get("username", "").strip()
+    password = body.get("password", "")
+    display_name = body.get("display_name", "").strip() or username
+    
+    if not invite_code or not username or not password:
+        raise HTTPException(status_code=400, detail="Invite code, username, and password required")
+    
+    if len(password) < 8:
+        raise HTTPException(status_code=400, detail="Password must be at least 8 characters")
+    
+    if not await use_invite_code(invite_code):
+        raise HTTPException(status_code=400, detail="Invalid or expired invite code")
+    
+    user = await create_user(username, password, role="user", display_name=display_name)
+    if not user:
+        raise HTTPException(status_code=400, detail="Username already taken")
+    
+    token = create_access_token(user["id"], user["role"])
+    return {
+        "token": token,
+        "user": {
+            "id": user["id"],
+            "username": user["username"],
+            "display_name": user["display_name"],
+            "role": user["role"],
+        }
+    }
+
+@app.get("/api/auth/me")
+async def get_me(user = Depends(get_current_user)):
+    """Get current user info."""
+    if is_single_user():
+        return {"mode": "single", "user": None}
+    
+    if not user:
+        raise HTTPException(status_code=401, detail="Not authenticated")
+    
+    return {
+        "mode": "multi",
+        "user": {
+            "id": user["id"],
+            "username": user["username"],
+            "display_name": user["display_name"],
+            "role": user["role"],
+            "has_api_key": bool(user.get("api_key")),
+        }
+    }
+
+@app.put("/api/auth/api-key")
+async def update_api_key(request: Request, user = Depends(get_current_user)):
+    """Update the current user's Lyria API key (BYOK)."""
+    if is_single_user():
+        raise HTTPException(status_code=400, detail="Not available in single-user mode")
+    if not user:
+        raise HTTPException(status_code=401, detail="Not authenticated")
+    
+    body = await request.json()
+    api_key = body.get("api_key", "").strip() or None  # Empty string = clear
+    
+    await set_user_api_key(user["id"], api_key)
+    return {"status": "ok"}
+
+# ─── Admin Endpoints ─────────────────────────────────────────────────────────
+
+@app.get("/api/admin/users")
+async def admin_list_users(admin = Depends(require_admin)):
+    """List all users. Admin only."""
+    users = await list_users()
+    return {"users": users}
+
+@app.post("/api/admin/users")
+async def admin_create_user(request: Request, admin = Depends(require_admin)):
+    """Create a new user. Admin only."""
+    body = await request.json()
+    username = body.get("username", "").strip()
+    password = body.get("password", "")
+    role = body.get("role", "user")
+    display_name = body.get("display_name", "").strip() or username
+    
+    if not username or not password:
+        raise HTTPException(status_code=400, detail="Username and password required")
+    if len(password) < 8:
+        raise HTTPException(status_code=400, detail="Password must be at least 8 characters")
+    if role not in ("admin", "user"):
+        raise HTTPException(status_code=400, detail="Role must be 'admin' or 'user'")
+    
+    user = await create_user(username, password, role=role, display_name=display_name)
+    if not user:
+        raise HTTPException(status_code=400, detail="Username already taken")
+    
+    return {"user": {
+        "id": user["id"],
+        "username": user["username"],
+        "display_name": user["display_name"],
+        "role": user["role"],
+    }}
+
+@app.post("/api/admin/invites")
+async def admin_create_invite(request: Request, admin = Depends(require_admin)):
+    """Generate an invite code. Admin only."""
+    body = await request.json()
+    max_uses = body.get("max_uses", 1)
+    expires_hours = body.get("expires_hours")
+    
+    code = await create_invite_code(admin["id"], max_uses=max_uses, expires_hours=expires_hours)
+    return {"code": code, "max_uses": max_uses, "expires_hours": expires_hours}
+
+@app.get("/api/admin/invites")
+async def admin_list_invites(admin = Depends(require_admin)):
+    """List all invite codes. Admin only."""
+    invites = await list_invite_codes()
+    return {"invites": invites}
+
+@app.delete("/api/admin/users/{user_id}")
+async def admin_delete_user(user_id: str, admin = Depends(require_admin)):
+    """Delete a user. Admin only."""
+    if user_id == admin["id"]:
+        raise HTTPException(status_code=400, detail="Cannot delete yourself")
+    
+    deleted = await delete_user(user_id)
+    if not deleted:
+        raise HTTPException(status_code=404, detail="User not found")
+    return {"status": "ok"}
+
+@app.put("/api/admin/users/{user_id}/role")
+async def admin_set_role(user_id: str, request: Request, admin = Depends(require_admin)):
+    """Set a user's role. Admin only."""
+    body = await request.json()
+    role = body.get("role")
+    if role not in ("admin", "user"):
+        raise HTTPException(status_code=400, detail="Role must be 'admin' or 'user'")
+    
+    updated = await set_user_role(user_id, role)
+    if not updated:
+        raise HTTPException(status_code=404, detail="User not found")
+    return {"status": "ok"}
 
 @app.get("/api/models")
 async def list_models():