A new scientific discipline has emerged in recent years, stimulated partly by advances in instrumental capability and partly by scientific discoveries. It is the study of systems of planets—their origins, existence, and properties—and of the interactive processes which link the planets with their central star. Two broad foci exist in this field—the first is the study of ‘‘other planetary systems.’’ For evolved systems, this means the detection and characterization of planets around the nearby stars, while for systems still in their natal phase it means the investigation of processes in regions where stars and planets may be forming. The second focus is the study of ‘‘our planetary system’’—the paradigm whose origin and evolution (both past and future) we wish to understand, and which affects us most directly. It is the thesis of this paper that the study of planetary systems will be a key discipline of the 21st century, and that it requires several observatories on the moon for its ultimate development.