Fiber optics have become one of the essential elements of modern optical technology. Early work has mostly focused on the transmission of light over long distances, particularly for use in optical fiber communications. Further work has greatly expanded the application areas of optical fibers, which now also include fields like fiber amplifiers, fiber lasers, supercontinuum generation, pulse compression, and fiber-optic sensors. This diversity of applications has been enabled by a variety of types of optical fibers, which can greatly differ in many respects. This Field Guide provides an overview of optical fiber technology. It not only describes many different types of fibers and their properties, but also presents in a compact form the relevant physical foundations. Sophisticated mathematics, e.g., concerning fiber modes, are not included, as such issues are covered in detail by several textbooks. Both passive and active (amplifying) fibers are discussed, and an overview on fiber nonlinearities and the application of active fibers in amplifiers and lasers is included. The large bibliography contains many useful references, covering both pioneering work and later seminal articles and books. This Guide should be very useful for a wide audience, including practitioners in industry as well as researchers.