The current state of research and development activities of optical intersatellite link (ISL) technologies in ATR Optical and Radio Communications Research Laboratories is introduced. Concerning modulation/demodulation techniques, the development of a high power, high speed optical modem is described. Optical beam control techniques under the solar conjunction are also introduced. Subjects for further research on coherent detection etc. are summarized. An optical space communication experiment program in USA, Europe, and Japan is outlined.