To implement a login system in Flask, you can follow these steps:

1. **Install Flask-Login**: Flask-Login is a widely used Flask extension that handles user authentication. You can install it using pip:

   ```bash
   pip install flask-login
   ```

2. **Set up User Model**: Create a User model to represent your application's users. This model should have fields like username, password hash, etc.

3. **Initialize Flask-Login**: Initialize Flask-Login in your Flask application:

   ```python
   from flask import Flask
   from flask_login import LoginManager

   app = Flask(__name__)
   login_manager = LoginManager(app)
   ```

4. **User Loader Function**: Implement a user loader function to load users from the database. This function is required by Flask-Login to manage user sessions:

   ```python
   from your_application.models import User

   @login_manager.user_loader
   def load_user(user_id):
       return User.query.get(int(user_id))
   ```

5. **Login View**: Create a login view where users can enter their credentials:

   ```python
   from flask import render_template, request, redirect, url_for
   from flask_login import login_user

   @app.route('/login', methods=['GET', 'POST'])
   def login():
       if request.method == 'POST':
           # Validate user credentials
           user = User.query.filter_by(username=request.form['username']).first()
           if user and user.check_password(request.form['password']):
               login_user(user)
               return redirect(url_for('index'))
       return render_template('login.html')
   ```

6. **Protect Routes**: Use the `@login_required` decorator to protect routes that require authentication:

   ```python
   from flask_login import login_required

   @app.route('/protected')
   @login_required
   def protected():
       return 'This is a protected route!'
   ```

7. **Logout**: Implement a logout function to allow users to log out:

   ```python
   from flask_login import logout_user

   @app.route('/logout')
   @login_required
   def logout():
       logout_user()
       return redirect(url_for('index'))
   ```

8. **HTML Templates**: Create HTML templates for login, logout, and other views as needed.

By following these steps, you'll have a basic user authentication system in your Flask application. Make sure to handle password hashing securely and consider additional security measures like CSRF protection.