WebSockets enable real-time, bidirectional communication between client and server. Unlike HTTP polling, WebSockets maintain a persistent connection for instant data delivery. The WebSocket protocol (RFC 6455) starts with an HTTP upgrade handshake. On the server side, use libraries like ws for Node.js or Django Channels for Python. Socket.IO extends WebSocket functionality with fallback options and rooms. Client-side implementation uses the API or Socket.IO client. Handle connection events: open, message, error, and close. Implement reconnection logic with exponential backoff. Use rooms or channels for targeted message delivery. Manage connection state for presence indicators. Implement authentication during the handshake phase. Scale horizontally using a pub/sub system like Redis. Handle backpressure to prevent overwhelmed connections. For broadcasting, use patterns like publish-subscribe. Monitor WebSocket connections for performance. Use compression for large payloads. Consider server-sent events for one-way real-time data. WebSockets are ideal for chat, live updates, gaming, and collaborative applications. Implement proper cleanup on connection close.