A fitting or adapter is used in pipe systems to connect straight sections of pipe or tube, adapt to different sizes or shapes, and for other purposes such as regulating (or measuring) fluid flow. These fittings are used in plumbing to manipulate the conveyance of water, gas, or liquid waste in domestic or commercial environments, within a system of pipes or tubes.

Fittings (especially uncommon types) require money, time, materials, and tools to install, and are an important part of piping and plumbing systems. Valves are technically fittings, but are usually discussed separately.