The potentially harmful effects of adverse environmental conditions during the transport of poultry and their welfare consequences are of political and public concern as well as being important in terms of increasing bird mortality and reducing product quality. The principal stressors during transport are listed, together with the relevant methods of assessing physiological stress. Particular reference is made to the thermal environment because of the major effects of thermal stress. Three-dimensional mapping of temperature and relative humidity within bird containers on commercial transport vehicles has been carried out. This has demonstrated large thermal gradients and the existence of a ‘thermal core’ within moving vehicles in summer, and in winter when side curtains are closed. Laboratory based simulations have resulted in the development of a model relating the degree of physiological stress expressed by the birds to the thermal load. This model will help in formulating recommendations for improved vehicle and container design and transportation practices.