In many regions in the U.S., load management and peak demand issues are of greater importance to utility planners than are reductions in energy consumption. Proper daylight utilization in commercial buildings can substantially reduce peak demand and increase energy savings. However, to determine optimum design strategies for controlling electrical demand, it is first necessary to understand the often conflicting impacts of fenestration on lighting and cooling loads. We use an hour-by-hour energy simulation model (DOE-2.16) to evaluate peak demand components and net effects in daylighted and nondaylighted buildings. More than 5000 parametric simulations were generated for prototypical office building modules containing both horizontal and vertical glazing, and located in 16 U.S. cities. From these simulations we draw conclusions about the effects of daylighting on peak demand for a range of climate types, orientations, fenestration areas, glazing shading coefficients and visible transmittances, U-values, lighting power densities, and lighting control strategies. Results for Los Angeles are briefly comparedsto results for the climatic extremes of Lake Charles, Louisiana (cooling-dominated), and Madison, Wisconsin (heating-dominated), and then discussed in detail. We also briefly describe studies in progress to measure peak load impacts of fenestration using an outdoor test facility and occupied buildings.