Implementation of computer software is concerned with trials of its robustness, relevance, and efficacy in the real-world, realtime context of design practice. This paper summarises the trials carried out within the Building Design Partnership of a dynamic energy model, ESP, developed by ABACUS at the University of Strathclyde. Over an 18 month period the program was used on 6 projects to address a variety of design problems over a variety of building types. The naper reports in outline on each of the six: four concerned with the need for a definite answer to a snecific question, eg. “will it overheat in summer” two concerned to provide, on the one hand for the client, on the other hand for the design practice, paradigms for energy conscious design of hospitals and offices. The conclusions drawn have relevance to the takeup of CAD generally.