Many contamination lessons have been learned since the Midcourse Space Experiment satellite was launched on April 24, 1996. FOremost is the inclusion of spacecraft design and thermal engineers with the contamination team early in the program, which resulted in the placement of vents away from the optical sensors, the separation into clean and 'dirty' sections, the exclusion of thrusters, the use of non- perforated silver/Teflon on the optical sensor axis, and the outgassing procedures instituted for all subsystem suppliers. The use of contamination instruments as monitors during integration and testing enabled correct technical decision to be made during several contamination incidents. In space, the contamination monitoring instruments supported programmatic decisions during the early mission planning. During the warm-up of the cryogenic telescope, it was learned that a small gap between the multilayer insulation and the internal baffles contributed to mass redistribution of water vapor. Consequently, it was also learned that a careful warming procedure may potentially be used to clean space-based cryogenic mirrors of condensed water vapor. Particles brought from the ground can be released by mechanical motions such as door openings as well as by thermal shock induced by the Sun during umbra exit. Solar- induced water evaporation from multilayer insulation can dominate the gaseous environment over the spacecraft's lifetime of several years.