The detailed assumptions and development of the nonlinear equations representing a reheat boiler-turbine-generator system are presented. The model is developed on the basis of a 'first principle' analysis and, therefore, model parameters are readily related to physical parameters. These data are generally available for such units. As a result of the nature of its development, the model better serves its intended purpose, i.e., control system design, as it yields physical insights which are valuable in formulating control system objectives.