While by now many museums have installed air conditioning, maintenance ranges in general from bad to appalling, to the extent of endangering the collection in many cases. Class 1 and class 11 air conditioning specifications are suggested, and the importance is stressed of correctly using two basic instruments: the light meter and the went-and-dry bulb hygrometer. There should always be a member of the conservation/curatorial staff with specific responsibilities as climate-control officer. Research should now be directed towards cheaper and more easily maintained systems of climate control than the present air conditioning systems.