In this chapter, microstrip patch antennas are thin and lightweight radiating elements, formed by a substrate, including one or several dielectric layers, backed by a metallic sheet (the ground plane). Thin metallic patches (the radiating elements) are located on the air-substrate interface and, possibly, between the dielectric layers. Microstrip antennas are manufactured by the photolithographic process developed for printed circuits. Their low profile, low weight and mechanical ruggedness make them an ideal choice for aerospace applications. They can be mass-produced, and could thus provide inexpensive receiver antennas for direct reception of microwave signals from satellites (television, mobile communications). Finally, they are ideally suited to be combined in large arrays, the individual patches sharing the same substrate. Thus directive antennas can be obtained in spite of the inherent low directivity of a single patch.