LDEF was carried into orbit in April 1984 by the Space Shuttle Challenger. The 11-ton satellite contained 57 experiments to assess the effects of the space environment, i.e., ionizing radiation, meteoroids, cosmic dust, and high altitude atomic oxygen on materials and mechanical, electronic, optical, and living systems. In January 1990, after 69 months in low Earth orbit, LDEF was retrieved by the Space Shuttle Columbia and returned to Earth. The retrieval occurred 57 months after it was originally planned, due in part to the Challenger tragedy. The 69 months in space provided experimenters the unique opportunity to sample and measure the space environment over a longer time period than originally planned. The 57 LDEF experiments were returned to the Principal Investigators and their science teams for analyses and interpretation. In June 1991, over 400 LDEF researchers and data users met in Kissimmee, Florida for the First LDEF Post-Retrieval Symposium. The papers presented contained important new information about space environments and their impact on materials, systems, and biology.