Mobile robots are key components in planetary exploration, to extend the area in reach for measurements, and to access points of scientific interest. After a review of particular design constraints of the space environment, a survey of planetary rover activities since the sixties is given. In particular, requirements and technical approaches of the actual small rover missions to Mars are discussed. The design of the actual NASA Mars microrover MFEX and the Russian Marsokhod are reviewed. In more detail the system and control concepts for the MARSNET mission of the European Space Agency (ESA) will be presented and discussed.