Importance of thermal radiation for transition to flashover in a compartment fire will be demonstrated. A one zone model is developed with the objective of capturing the key mechanisms responsible for the initiation of flashover, i.e. physics of flashover. Thermal radiation is shown to be a key point and the inclusion of a three-dimensional radiation model is essential in the development of an accurate understanding of flashover. The heat transfer from the bounding wall of the compartment, the particulate concentration in the hot layer and the ventilation opening are shown to be key parameters which can lead to thermal instability and the resulting initiation of flashover. (C) 2004 Elsevier Ltd. All rights reserved.