This report summarizes the research and development undertaken at FOI in the frame of free-space optical communications (FSO) 1999-2007. FSO is an alternative to traditional radio or fiber-optical communication. Communication at high transfer rates (more than 10 Gbit/s) is possible and the links can be mobile. Furthermore, the optical beam is usually narrow and directed, which makes it difficult to eavesdrop, jam or even detect the communication. FOI have studied methods particularly applicable to tactical applications. The main focus has been on communication based on modulated retroreflectors (MRR). Such asymmetric links have a laser transceiver in one end and an MRR assembly in the other. A MRR consists of a retroreflector with an optical modulator. The laser transceiver transmits a continuous wave laser beam towards the MRR which modulates the optical beam and reflects it back to the transceiver. Information can be sent in the opposite direction by modulating the transceiver output beam. The main advantage with the MRR technology is that complex beam steering is avoided. Moreover, small and lightweight installations with low energy consumption can be envisaged. Different types of optical modulators, based on quantum well structures in semiconductors or glass cells with liquid crystals have been studied. Several laboratory systems were tested and evaluated in laboratory trials and in field trials, e.g. signals have been transmitted from a buoy in water, carrying an MRR assembly, to a transceiver on the shore. Some examples of other activities within the field of FSO are non-mechanical beam steering with spatial light modulators (SLMs), air jump between optical fibers, non line-of-sight communication and quantum key distribution (so-called quantum cryptography).