There is a need to engineer working areas where air-borne pollution is below specified limits. This need arises from activities as diverse as electronic component manufacture, food processing, the production of pharmaceuticals and the handling of hazardous materials. In this paper, detailed experimental measurements of flows and temperature distributions in clean rooms are presented. Computational Fluid Dynamics (CFD) is used to predict velocity and temperature profiles in the room. The computational predictions are compared with detailed experimental data taken in a room in which controlled forced ventilation and a heat source were imposed. The predictions show good agreement with the measurements. They also show some interesting features which can help the architects and clean room designers achieve working environments with higher specification without making unacceptable demands on the forced ventilation systems. The CFD work has been extended to provide ’figures of merit’ of the efficiency of the ventilation within a room. These figures can help the designers to evaluate their systems quickly and efficiently, they can also be used to compare the thermalhydraulic performance of different clean rooms.