Currently, in excess of 650 million broiler chickens are produced per annum in the UK.The rearing of these animals on large numbers of geographically dispered sites necessitates their transportation to centralised processing plants for slaughter. Birds are transported in containers in which behaviour, including any thermoregulatory component, will be restricted. The design of the container and the vehicle and the external climatic conditions will have profound effects upon the immediate environment of the birds. In transit they may be exposed to a variety of potential stressors including the thermal demands of this transport microenvironment, motion, acceleration, vibration, impacts, fasting, withdrawal of water, social disruption and noise. The adverse effects of these factors and their combinations may range from discomfort and mild aversion to death. Mortalities in transit are generally 0.4% or less but this may represent approximately 2 million birds per annum in the UK. It is estimated that up to 40% of the mortalities observed at the processing plant are a consequence of “stress”. It is thus essential from the standpoints of both animal welfare and productivity to optimise commercial poultry transport conditions and to provide the sound scientific basis for legislation relating to vehicles and transport practices. Although examination of the existing literature reveals that thermal stress is acknowledged as a major hazard during animal transportation it has been poorly characterised under practical conditions and the interactions between the animals and the complex thermal microenvironments clearly require more rigorous analyses.