Even before Louis Sullivan coined the phrase `Form Follows Function,'architectural researchers have sought, to no avail, a causal relationship between these two primary constituents of the building enterprise. This paper attempts to explain why this quest has been futile, and proposes a performance-based design paradigm, instead of the prevailing process-based paradigms. It suggests that the driving force behind any design activity is the desire to achieve a qualitative solution for a particular combination of form and function in a specific context. Furthermore, it suggests that quality can only be determined by a multi-criteria, multi-disciplinary performance evaluation, which comprises a weighted sum of several satisfaction/behaviour functions. The paper develops a performance-based design methodology and demonstrates its application in an experimental, knowledge-based CAD system.