Critically examines the history and development of museum climate recommendations during the 20th century. The authors find that initially there was only minimal scientific support for the environmental values and ranges selected, and that not enough effort was put into establishing the effects of environment on materials and objects. “Best guesses” were subsequently codified as best practices, although they often had less to do with the preservation of objects than with the mechanical limitations of HVAC systems, constraints imposed by the exterior climate or historic building regulations, or implementation and maintenance costs. Recent research has provided a general scientific basis for determining appropriate museum climate values, and in particular, safe variation or fluctuation ranges for temperature and relative humidity. These results were initially criticized, since they differed from earlier practice, but they have proven valid over time and are now increasingly accepted.