A method of communicating data between devices, typically a computer and its peripherals, the individual data bits being sent sequentially. Serial communication may be asynchronous, where the data characters include start and stop bits to delimit the data, or synchronous, where such additional bits are omitted and the delimiting of the data depends purely on timing. Asynchronous serial communication is more flexible whereas synchronous serial communication makes better use of the available bandwidth. Asynchronous methods are generally used with dial-up modems or for general connection of simple serial peripherals. Synchronous methods are usually to be found where leased lines or proprietary interfaces are used.

Formerly SIO was a relatively slow mode, but advances in silicon technology mean that very fast SIO (hundreds of megabits per second) now make SIO preferable to parallel interfaces. See USB, Firewire.