An extensive literature survey has been made of the formation mechanisms involved in NO generation within the flame front (“prompt NO”) and in the post-combustion gases. Computer models having a uniform temperature and composition in the burnt charge are showm to be inaccurate in predicting NO. Improvements can be achieved by the incorporation of a one-dimensional temperature gradient and partial mixing in the burnt gases. However, the development of models in which heat transfer is confined to a multi-element “thermal boundary layer” is required for the most accurate predictions. Limited results from one such model are presented.