In this paper we discuss results from a number of parametric analyses of thesenergy and cost influences of fenestration in a prototypical office building.sThe energy important parameters of fenestration, daylighting, and electricslighting were systematically varied in several climates using the DOE-2.1senergy simulation program to determine net annual results. Results arespresented for two climate extremes; one heating-load dominated and the otherscooling-load dominated. The increase or decrease of net annual energy consumptionsand peak electrical demand due to fenestration is demonstrated. Daylighting,sis shown to be the single most important strategy to reduce energysuse, but can be an energy and cost liability. Conditions under which thesesliabilities occur are discussed, and optimal design solutions for minimizingsenergy costs are suggested.