The passive cooling of buildings by radiative heat loss has received comparitively little attention in comparison with passive solar heating. This is due, in part, to the more limited heat fluxes available, with consequently more severe constraints on system design. This work examines these constraints, formulates design guidelines, and describes the design, modelling and experimental testing of a new passive radiative cooling system for buildings.The passive cooling system described in this work utilises the thermal diode action of closed two-phase therrnosyphons (gravity - assisted heat pipes), and an associated thermal store, to reject heat from a building through an insulated roof. A computer model of the system was developed to identify the potential of the system. The model utilises a detailed resistance network and a lumped parameter approach, and includes the effects of heat pipes cycling on and off, and the melting and freezing of a latent heat store. The results of the modelling work indicated that a heat- pipe- oased roof module could 0 lower the internal temperature of a heated building by 5 - 6 oC, dependent on the heat load and external environmental conditions.A facility was developed for the manufacture and testing of heat pipes for the passive cooling application. This facility included a heat pipe filling and degassing rig, and computer controlled performance test rig. A previously unreported heat pipe "startup" phenomenon was identified, and found to be related to the heat pipe shape, internal surface treatment and driving temperature potential ∧T. An evaporator insert was used to overcome this problem, so that the heat pipes then exhibited a linear increase in transmitted heat flux with ∧T. A computer model of the heat pipe performance was developed, and found to be in good agreement with the measured data. The fill of methanol working fluid was· optimised at 30 - 40 % .An assessment of candidate phase change thermal storage materials was carried out, and straight chain alkyl hydrocarbons (low melting point paraffin waxes) identified as ideal for the passive cooling application. Several samples were obtained and tested.A detailed assessment of the potential for radiative cooling in Brisbane was carried out. An automatic weather station was set up with the inclusion of a pyrgeometer for the measurement of incoming sky radiation. This enabled calculation of the effective sky temperature, so quantifying the potential for radiative cooling of buildings as a weighted measure of the effects of cloud cover and humidity. 285,000 readings of atmospheric parameters were analysed, and the data summarised as monthly graphs of effective sky temperature, and depression of the sky temperature below ambient.An optimised passive radiative cooling system incorporating heat pipes and a ceiling based thermal store was manufactured, and tested in an outdoor test facility developed by the author. This facility allows the concurrent testing of two complete roof modules at variable (computer controlled) internal heat 0 loads. The flat plate thermal diode module was found to typically cool 4.5 oC below a non - heat - pipe control module under "no load" conditions, and 9.8 °C lower with a 47 Wm-2 internal load . The system brought the mean internal air temperature below that of the external ambient mean, and reduced the diurnal swing from 11. 0 C to 4 .1 oC.The heat pipe thermal diode radiative cooling system was thus found to provide excellent coupling between the internal room air and the external radiative sink, and an efficient automatic thermal diode action which screens out daytime heat gain. The limiting factor in the application of the concept is ultimately the radiative cooling resource at the particular location. The system performance will be maximised at locations characterised by clear skies and low humidity.