The crystal structure of ferrites including garnets and their methods of preparation are first reviewed. This is followed by an examination of microwave ferrite devices and their application to actual systems. Sufficient theoretical details as well as an extensive though not exhaustive list of references are included for ready reference. The most widely used microwave ferrite device is the isolator, a device which possesses nonreciprocal properties. The function of this device in various configurations is therefore studied in some detail. Then, ferrite-phase shifters, including the latching variety, circulators, modulators, power limiters, switches, amplifiers, delay lines, and filters are examined in turn. Microwave ferrite devices, in particular isolators and circulators, are commonly used in the laboratory. Other devices, such as modulators, limiters, and switches, are used respectively in microwave systems in carrier modulation, receiver protection, and antenna switching, etc. Much of the ferrite device research and development in recent years has been concerned with the -junction circulator and the coincidence power limiter.