The progress of research aimed at improving these two cells so as to withstand the rigours of the space environment is reviewed using as a basis the proceedings of the photovoltaic specialists conferences which have been held throughout the last decade. The present position with the two cells is compared. Finally the manufacturing costs of the cells are briefly compared and it is concluded that CdS cells will about one third of the price of silicon cells. Furthermore it is considered that this cost differential will be maintained provided that the production engineering of both types is advanced in parallel