CAT5 — sometimes referred to as CAT 5 or Category 5 — is an Ethernet networking cable standard defined by the Electronic Industries Association and Telecommunications Industry Association. CAT5 cables use the fifth generation of twisted-pair Ethernet technology and have become the most popular of all twisted-pair cable types since their introduction in the 1990s.

CAT5 cables contain four pairs of copper wire that support Fast Ethernet speeds of up to 100 Mbps. As with all other types of twisted-pair EIA/TIA cabling, CAT5 cable runs are limited to a maximum recommended run length of 100 meters (328 feet).

While CAT5 cables typically use four pairs of copper wire, Fast Ethernet communications use only two pairs. In 2001, the EIA/TIA published a newer Category 5 cabling specification called CAT5e (or CAT5 enhanced), designed to better support Gigabit Ethernet speeds of up to 1000 Mbps by using all four pairs of wires. CAT5e cables also maintain backward compatibility with Fast Ethernet equipment.

While not technically rated to support Gigabit Ethernet, CAT5 cables are capable of supporting gigabit speeds over shorter distances. The wire pairs in CAT5 cables are not twisted as tightly as those constructed to CAT5e standards, and thus have a higher risk of signal interference that increases with distance.