Plans for extended duration missions, such as a Mars expedition and outpost, could involve a small crew working for up to 3 years in isolated, confined, and potentially dangerous environments. Behavior and performance studies conducted on crew during short-duration flights have mainly focused on how the space environment affects physical performance. These studies concluded that after a short adaptation period, human behavioral capabilities can be maintained during short-duration space flights. For long-term missions, scientists need a greater understanding of psychological, social, environmental, perceptual, and behavioral aspects of human reactions to the space environment. Crew complaints, such as poor sleep quality, high noise levels and unpleasant odors in the spacecraft, lack of privacy, and lack of diversity in their food must also be addressed in planning extended-duration space flights. In the era of the International Space Station, bridging communication gaps between crew who speak different languages and operate under different styles of leadership will be extremely important to the success of the missions. Key concepts include: stress, stressor, habitability factors, human performance, isolation and confinement, teamwork and group solidarity.