Small Computer System Interface, is a set of evolving ANSI standard electronic interfaces that allow personal computers to communicate with peripheral hardware such as disk drives, tape drives, CD-ROM drives, printers, and scanners faster and more flexibly than previous interfaces. Developed at Apple Computer and still used in the Macintosh, the present set of SCSIs are parallel interfaces. SCSI ports are built into most personal computers today and supported by all major operating systems.

In addition to faster data rates, SCSI is more flexible than earlier parallel data transfer interfaces. The latest SCSI standard, Ultra-2 SCSI for a 16-bit bus can transfer data at up to 80 megabytes per second (MBps). SCSI allows up to 7 or 15 devices (depending on the bus width) to be connected to a single SCSI port in daisy-chain fashion. This allows one circuit board or card to accommodate all the peripherals, rather than having a separate card for each device, making it an ideal interface for use with portable and notebook computers. A single host adapter, in the form of a PC Card, can serve as a SCSI interface for a "laptop," freeing up the parallel and serial ports for use with an external modem and printer while allowing other devices to be used in addition.

Although not all devices support all levels of SCSI, the evolving SCSI standards are generally backwards-compatible. That is, if you attach an older device to a newer computer with support for a later standard, the older device will work at the older and slower data rate.

The original SCSI, now known as SCSI-1, evolved into SCSI-2, known as "plain SCSI." as it became widely supported. SCSI-3 consists of a set of primary commands and additional specialized command sets to meet the needs of specific device types. The collection of SCSI-3 command sets is used not only for the SCSI-3 parallel interface but for additional parallel and serial protocols, including Fibre Channel, Serial Bus Protocol (used with the IEEE 1394 Firewire physical protocol), and the Serial Storage Protocol (SSP).

A widely implemented SCSI standard is Ultra-2 (sometimes spelled "Ultra2") which uses a 40 MHz clock rate to get maximum data transfer rates up to 80 MBps. It provides a longer possible cabling distance (up to 12 meters) by using Low Voltage Differential (LVD) signaling. Earlier forms of SCSIs use a single wire that ends in a terminator with a ground. Ultra-2 SCSI sends the signal over two wires with the data represented as the difference in voltage between the two wires. This allows support for longer cables. A low voltage differential reduces power requirements and manufacturing costs.

The latest SCSI standard is Ultra-3 (sometimes spelled "Ultra3")which increases the maximum burst rate from 80 Mbps to 160 Mbps by being able to operate at the full clock rate rather than the half-clock rate of Ultra-2. The standard is also sometimes referred to as Ultra160/m. New disk drives supporting Ultra160/m will offer much faster data transfer rates. Ultra160/m also includes cyclical redundancy checking (CRC) for ensuring the integrity of transferred data and domain validation for testing the SCSI network.