The solar cells used in space for over 40 years are reviewed by discussing the semiconductor materials which have provided the best cells. Most emphasis was on high efficiency, combined with good tolerance to charged particle bombardment, and the steady increase in efficiency is discussed. The most important requirement is that the cells must be highly reliable, consistent in performance, and stable while operating in space. The need for highest reliability makes the costs less important. The progress to date has provided a good foundation for future applications for space cells. (C) 2001 Elsevier Science B.V. All rights reserved.