The Field Museum of Natural History, Chicago, has implemented the use of relative humidity modules to control relative humidity in display cases containing moisture-sensitive objects. The humidity control module developed at the Canadian Conservation Institute has been used to control two cases since 1987. Problems encountered with its performance over 10 months led to modifications of the module. Three more modules were installed in 1988 in a larger, more complicated exhibit. This article outlines the development and modification of the modules and their performance over four years.