Frequency selective limiters (FSL) are extremely useful in today’s dense signal environment. They allow wideband receivers to process weak signals without being overloaded or damaged by the presence of strong signals. FSLs are completely passive devices that make use of the nonlinear behavior of ferrites. This presentation will give the basic theory of the nonlinear excitation of spin waves that absorb RF energy above a certain critical threshold level. An explanation as to how this absorption occurs only at frequencies close to that of a coincident strong signal will be given. FSLs based on the absorption of excess power in ferrite-loaded transmission lines and those based on the saturation of propagation of magnetostatic waves (MSW) will be discussed. Design tradeoffs and ferrite material requirements will be considered. A survey of the operating characteristics of a wide variety of lumped and distributed transmission line FSLs and MSW FSLs will be given.