Lanthanide ion-doped fibre optic devices have numerous applications. Commercially available fibre lasers cover wavelengths spanning from the visible range up to 2 μm, while mid-infrared fibre lasers are being developed in the research labs. When compared with other types of lasers, fibre lasers are characterized by a high quality of output beam and ease of beam delivery. Apart from fibre lasers, optical fibre amplifiers are of large practical importance. These devices play a prominent role in long haul telecom systems as signal regenerators.