Magnesium-matrix composites (MgMCs) are very promising as structural materials because of their low density, high specific strength, and excellent castability. This article provides information on the characteristics, mechanical properties, and applications of magnesium alloys and composites. It discusses the microstructures used for the most common magnesium alloys used in metal-matrix composites, namely, magnesium-aluminum, magnesium-rare earth and magnesium-lithium alloys. The article focuses on the most common methods of heat treatment, including solution heat treatment, precipitation strengthening or aging, and annealing, applied to these alloys. Finally, it describes the microstructural aspects and precipitate-matrix relationships of MgMCs as well as the heat treatment methods for MgMCs.