Passive and low energy applications in buildings have become a topic of worldwide interest within the last few years. It has now been demonstrated very clearly that indoor comfort can be maintained with an expenditure of only 10 to 20% of the energy often required by modern buildings. This is accomplished through a combination of conservation measures to minimize the load, passive use of solar energy for heating, natural cooling, and daylighting. The major research emphasis has been on devising mathematical models to characterize heat flow within buildings, on the validation of these models by comparison with test results, and on the subsequent use of the models to investigate the influence of both design parameters and weather on system performance. Design guidelines have been developed, and simplified methods of analysis have been promulgated. Performance has been monitored in test modules, test buildings, and many residential and commercial buildings. The results both confirm good performance and establish the accuracy of model predictions. A significant change in the research picture has been seen in the last 4 years; whereas the major effort was originally in the United States, research is now being conducted in many countries throughout the world as many people have realized that passive and low energy methods are appropriate in virtually every climate and are well suited to economic, convenient, and reliable building construction and operation.