It is now accepted by many that for transmission systems operating at 1.55 μm, erbium doped optical fiber amplifiers are superior to semiconductor diode amplifiers. They have higher fiber-to-fiber gain, lower crosstalk and noise, are polarization-independent and do not suffer from facet reflections. The tutorial covers fiber fabrication, the spectroscopy of erbium in glass, and fundamentals of amplifier behavior. The available pump options are discussed, together with achievable gain and noise performance. Uses and systems experiments are reviewed and future developments explored.