The main objective of this chapter is to introduce platforms and sensors commonly used in remote sensing systems for various applications. Platforms of interest in this chapter include space-borne, air-borne, sea-based, and ground-based platforms. Both active and passive sensors may be installed on platforms to provide a variety of remote sensing systems while satellite constellations enable the networking capacity of different sensors and platforms. Some of these sensors may have common functionalities in current, future, and historic missions for prolonged Earth observation.