BACKGROUND As we move into the next Millennium, increasing numbers of people will travel into space. Psychological screening methods will be relaxed on near-Earth missions, such as might occur on a space station or a lunar colony. Crewmembers on interplanetary missions such as a trip to Mars will have to deal with psychiatric problems themselves with no possibility of evacuating an affected individual. For these reasons, it is important for support personnel on Earth and crewmembers in space to be knowledgeable about psychiatric difficulties that might occur and their appropriate treatments. METHODS Anecdotal reports from long duration manned space missions and studies from space analog environments on Earth were reviewed for information concerning these issues. RESULTS Psychiatric problems pertinent to the space environment include adjustment and psychosomatic reactions, asthenia, mood and thought disorders, and post-mission personality changes and family problems. Counter-measures to ameliorate these difficulties involve pre-launch selection and training; monitoring, support, and counseling/psychotherapy during the mission; and post-return debriefings with crew members and their families. Psychoactive medications have been useful during space missions, although unclear pharmacokinetic issues related to microgravity need to be taken into account. CONCLUSIONS Although much is known about psychiatric issues related to long duration manned space travel, more empirical work needs to be done during actual space missions.