Despite the proliferation of the usage of simulation programs in the conventional building design process, the use of thermal simulation programs in assessing energy and thermal performances of building designs are very limited. This thesis aims to investigate the role of thermal simulation programs in various phases of building projects‟ life cycles, and evaluate potential benefits these programs can bring. To illustrate the potential of these simulation programs, two case studies, one from the schematic design phase and the other from the post-occupancy phase, are presented. The thesis shows that, at each of these phases, thermal simulations can act as a feedback tool where performances of different strategies can be evaluated, which in turn allows energy conserving strategies to be recognized. The thesis also shows that, at the post-occupancy phase, conducting thermal simulation studies with existing buildings can also help highlight poor performances.