How does Node.js handle concurrency without multiple threads?

Node.js achieves concurrency through its event loop and non-blocking I/O without using multiple OS threads for request handling. The key insight is that most server work is I/O-bound (waiting for databases, file system, network) rather than CPU-bound. While waiting for I/O, a thread in traditional models is blocked doing nothing. Node.js instead uses the OS's asynchronous I/O capabilities (via libuv): when an I/O operation is initiated, Node.js registers a callback and immediately returns to the event loop to handle other work. The OS notifies Node.js when I/O completes (via epoll, kqueue, or IOCP depending on the OS), and the callback is queued for the event loop to execute. This means a single thread can juggle thousands of concurrent I/O operations simultaneously — each "in flight" while the thread handles other requests. The cost: CPU-intensive operations (complex computations, synchronous operations) block the single thread, halting all other requests. Solutions: Worker Threads, child_process, or offloading CPU work to external services.