Communications in space utilizing lasers has been considered since their realization in 1960. It was soon recognized that, though the laser had the potential for the transfer of data at extremely high rates, much work was required in system and component development, particularly for space-qualified hardware. Advances in system architecture, data formatting, and component technology over the past quarter century have now made laser communications in space a viable and attractive approach to space based communications. In particular, semiconductor laser diodes offer important advantages over other laser sources, in size, weight, volume, efficiency, and data rate. There are however, performance restrictions which impact the system design and the direct application of laser diodes for communications in space. This paper presents an overview of space-borne laser communication systems and key laser diode parameters which affect their application.