What is UART flow control and when is it needed?

Flow control prevents data loss when receiver cannot keep up with transmitter. Hardware flow control (RTS/CTS): RTS (Request To Send) signals receiver readiness, CTS (Clear To Send) acknowledges; transmitter pauses when CTS inactive. Software flow control (XON/XOFF): Special characters signal start/stop transmission; simpler but reduces throughput and has latency. Flow control is needed when: baud rates are high, receiver processing is variable, buffers are limited, or data integrity is critical. Modern systems often use DMA and large buffers instead.