We present results from the first phase of a project to develop a fenestration performance design tool to besused by builders, designers, architects, utility auditors, etc. In phase 1 we defined the design tool conceptsand the experimental and analytical methodologies required to achieve the project goal. We defined fivesfenestration performance indices, which when combined with user-specified weighting factors yield a singlesfigure of merit. Three of the indices are related to the effects of fenestration on building energy performance:sfuel and electric use and peak electric demand. The other two are related to thermal and visualscomfort. We derived index values and correlations to window design parameters by creating a data basesconsisting of a large number of building energy simulations for a prototypical o5ce building module usingsthe DOE-2 computer simulation program. Four glazing types and two shading devices were combined insseveral ways so that a representative sampling of realistic fenestration systems was analyzed.