This paper describes an investigation into the use of building thermal capacitance as a means of reducing the operating costs associated with maintaining adequate comfort conditions in buildings (termed "dynamic building control"). The state of the building thermal storage can be controlled through variations of the zone temperatures over time within the thermal comfort region. The primary opportunities in varying zone setpoints in an optimal fashion are associated with shifting cooling loads from daytime to nighttime to (1) reduce peak electrical demands, (2) take advantage of low nighttime electrical rates, (3) offset mechanical cooling with "free" cooling at night, and (4) enhance equipment operation at more favorable part-load conditions. The approach utilized in this study was to apply dynamic optimization techniques to computer simulations of buildings and their associated cooling systems for a range of conditions in order to determine the maximum possible savings. Results indicate that both energy costs and peak electrical use can be significantly reduced through optimal control of the intrinsic thermal storage within building structures. However, the cost savings depend strongly on several factors including 1) utility rate structure, 2) part-load characteristics of the cooling plant and air handling system, 3) weather,4) the occupancy schedule, and 5) building thermal capacitance.