The principal characteristics of gyromagnetic materials that are useful for microwave applications are reviewed, and both the large-signal and small-signal models that govern wave propagation at microwave frequencies are given. Uniform and nonuniform plane waves in an unbounded ferrite medium are considered from the complementary viewpoints of electromagnetics and mechanics. Both electromagnetic and quantum-mechanical exchange channels of power exist in such materials, which can be ascribed to either waves or quasiparticles. Regimes of wave propagation that are magnetostatic in character are shown to exist, as well as the relationships between the Walker modes of a small spheroid and the magnetostatic wave propagation in thin films. When the power densities within these waves or modes exceed certain thresholds, the linear model breaks down and parametric instabilities can create a form of magnetic turbulence. The thresholds for both first-order and second-order processes that involve the uniform precession mode and parallel-pumping process that do not are reviewed.>