The determination of an optimal value of relative humidity for the preservation of museum objects is more complex than for other environmental factors. Relative humidity affects the preservation of objects in many ways, and effects vary for different types of object. Relative humidity affects the rates of chemical reactions, and the values of physical properties such as size, strength, and stiffness. Extremes of either low or high relative humidity can be damaging. Changing the relative humidity may decrease damage due to one factor while increasing damage due to others. This article examines ways in which relative humidity affects the degradation processes of different types of material, the limits each factor places on allowable values of relative humidity, and how each factor influences preservation within the allowable range. The type of collection, its use, and constraints imposed by the building housing the collection are also considered. The optimal relative humidity is not a specific value upon which all considerations converge, but a range chosen as a compromise in an attempt to minimize the total effect of numerous reactions and processes.