A variety of optical components have been made directly from single-mode fibers. These components can be combined to construct in-line all-fiber systems such as fiber gyroscopes, fiber Raman lasers, local area networks, or coherent lightwave systems. This paper reviews the present state of passive and active single-mode fiber components. Passive components are polarizers, directional couplers, filters, and Faraday rotators. Active components require external control or optical power and include modulators, polarization controllers, frequency shifters, and amplifiers. Polarization is usually important in fiber components and discussions of polarization-maintaining and ordinary single-mode fibers are included.