Much effort has been devoted over the years to advance Building Performance Simulation (BPS) by improving algorithms and by extending the simulation domain to daylighting, acoustics, and indoor air quality. Yet in several recent relevant ASHRAE forums many attendees asked for transparency, useability, and flexibility of computer programs. The issues of flexibility, transparency, and ease of use are categories commonly associated with the user interface. Moreover, they relate fundamentally to software architecture. A good user interface is the foundation of a program, not a finishing touch. While seemingly unrelated to BPS, the tedious "bookkeeping" tasks of data entry, data management, and reporting occupy a major part of the user's time and should similarly be near the top of the software developer's priorities. How well these tasks are handled is the key to usability, flexibility, and transparency of a BPS package. The budgets of most software developers in the area of BPS are limited and usually do not allow the type of development effort necessary to deliver the quality of user interface today's more sophisticated users demand. This paper describes an approach that relies heavily on commercial software packages, such as database managers, spreadsheets, and wordprocessors. Besides yielding more product bang for the development buck, this approach goes a long way to making BPS more flexible, transparent, and less onerous for the user. The software architecture of a typical BPS system is presented with detailed indications as to which tasks can be handled by commercial software packages, and which are likely to require programming by the specialist. The capabilities of several types of popular commercial software packages are presented as they might apply to the needs of BPS: Database managers can maintain building component libraries, such as records of envelope assemblies, equipment performance specifications, HVAC control settings; Wordprocessors with merge and sub-document inclusion capabilities can be harnessed to become automatic report generators; Spreadsheets and graphics packages can represent simulation results in graphic form. In our opinion, only the overall integration of all software components and the real, numbercrunching aspect of Building Performance Simulation should be handled by specialized routines written in high-level code. The challenge to the software developer is the successful integration of commercial software with in-house developments. To the user, well integrated systems look simple, uniform, and provide the required flexibility and transparency.