A review of current daylighting design tools indicates that over 30 tools are now available, including nomographs,protractors, overlays, and programs for micro-, mini-, and mainframe computers. Computer-based tools allow testing and analyzing of more design alternatives under a wider variety of conditions than was previously practical with slower techniques, but they have not fundamentally changed the information available to the design professional. These tools are applicable to certain limited functions of the design process and do not address the varying information needs of the designer at various points in the design process. We describe a concept for a computer-based building envelope designstool that is structured to overcome many of the limitations of the existing tools. The tool would address daylighting design in the context of the overall building envelope design, covering a range of environmental quality issues in addition to quantitative aspects of lighting and energy use. It would be useful throughout the process of design, construction, and occupancy, and is intended to provide important feedback that is often missing between those stages of the buildings life cycle. A cost-effective tool with these performance features is not technically feasible using todays hardware technology. However, examination of the development of the necessary technologies provides strong evidence for future feasibility; accordingly we are developing the tool to be used in a 1990+ time frame. To date we have studied the features and capabilities that such a tool should have, as well as several key areas, such as the design process, computer graphics, imaging systems, expert systems, and building science data bases.