These materials were designed to be used by life science students for instruction in the application of physical theory to ecosystem operation. Most modules contain computer programs which are built around a particular application of a physical process. This report introduces two models of the thermal energy budget of a leaf. Typical values for environment variables and leaf parameters are discussed and simple calculations are made to see how radiation, convection, and transpiration affect leaf temperature. A graphical method of analysis is used to present a more detailed energy budget model. A problem set and an accompanying computer program called TRANS permit the student to explore the consequences of the models. Algebra and some knowledge of heat transfer physics are prerequisites. (Author/CS)