Even before the first satellites were launched into Earth orbit scientists and engineers became interested in carrying out missions to the Moon and to the planets Venus and Mars. There have been many setbacks in their efforts, but also many successes. This chapter summarizes our exploration of the solar system using unmanned spacecraft, plus the science undertaken by the Apollo missions. The preparations for a renewal of human exploration beyond low Earth orbit must take into account both the physical and psychological challenges which human beings will face in making such journeys. Projects addressing the psychological challenges of isolation are described, such as the recently concluded Mars 500 simulation of a mission to Mars.