The only way to describe the effects of the thermal environment on the human body completely is to do it by means of an energy balance equation. In such an equation all relevant meteorological parameters, behavioral characteristics (activity and clothing) and body measurements can be considered. Using Fanger's comfort equation and the models MEMI and IMEM as examples, the problems of energy balancing and ways of solving them are described. The value of energy balance models is documented by examples from the field of application.