The space environment is hostile to people and equipment. In this chapter the space environment is considered in terms of separate categories: gravity, neutral particles, vacuum, plasma, radiation, micrometeoroids, and space debris. The microgravity environment available in space is an interesting laboratory for researchers in many disciplines but as a workplace it poses physiological challenges to humans. Weightlessness can be produced for short periods of time in ground-based experiments in drop towers, parabolic flights on board aircraft, and sounding rockets. One of the most challenging aspects of a long-duration space mission for human exploration of the solar system is the susceptibility of both people and equipment to ionizing radiation consisting of relatively constant levels of galactic cosmic rays and sporadic high-energy solar particle events. Our current knowledge is insufficient to estimate with certainty the risk to astronauts from an extended period of exposure to this radiation. Since the space age began, spacecraft have been launched with little concern for the resulting pollution. As a result the most popular Earth orbits are littered with the debris of space missions, and this puts new missions at risk of potentially damaging collisions.