During the summer, buildings in hot dry climates have the inevitable problem of cooling. These climates are characterized by hot summer days with cold nights, a high degree of solar radiation, low humidity and with a nearly fixed seasonal and daily pattern of wind. These natural phenomena could be exploited by nocturnal ventilation to cool the building fabric, thus saving energy during the day and providing comfort at night. The procedures to evaluate thermal performance of buildings with special reference to nocturnal ventilation are studied. Various approaches to building thermal response are first reviewed. Dynamic thermal simulation computer models are developed to predict hourly 'internal temperatures'. These are used to study the various constituents of models. They are based on: -the Admittance Method (as suggested by the CIBSE Guide); -a similar procedure but with higher harmonics; -the Response Factor Method (suggested by ASHRAE); -and the Finite Difference Method. A room surrounded by similar rooms in a multi-storey building, having only one external wall, was simulated in the laboratory. It was subjected to typical variations of a hot climate. Predictions of the computer simulations are compared with laboratory results and it is shown that -the closest agreement was obtained with the Response Factor and Finite Difference methods which are equally good; -for higher rates of ventilation, representation of a room by a simple three nodes model thermal network will give sufficiently accurate results; while for lower rates of ventilation a more detailed model gives more accurate results; -the standard Admittance Method gives adequate results, especially with higher rates of ventilation. It could also be used for hourly temperature-, calculations and variable ventilation without loosing significant accuracy; -a fuller treatment in the Admittance Method of time-lag and time-lead, associated with the dynamic thermal factors, will not greatly improve the results. An increase in the number of harmonics in the procedure did not also result in significant improvements, especially with a high rate of ventilation. Natural ventilation into rooms through open windows in these climates is theoretically investigated. It is shown that the rate of natural air flow obtained may be sufficient to meet the requirements of passive cooling by nocturnal ventilation. A computer program is developed to calculate the rate of air flow in multi-zone buildings, and a new relationship is suggested, which will reduce the complexity of natural air flow calculations in multi-zone buildings subjected to cross ventilation.