Whereas previous modules dealing with heat energy have introduced physical principles (Gates and Stevenson 1978; Stevenson 1977a,b, 1978a) or methods of analysis (Ratheway 1978, Stevenson 1978b), the first section of this paper will emphasize animal thermoregulation from an ecological viewpoint. Physiologists have investigated thermoregulation more than ecologists, so the student will find the best summaries in environmental physiology texts such as Gordon (1977) and Schmidt-Nielsen (1975) or summary reviews (Whittow 1970, 1971, 1973; Precht et al. 1973; Wieser 1973). Gibbons and Sharitz (1974) and Gates and Schmerl (1975) also contain papers which are of interest to students of thermal biology. [UPDATE: Thermoregulation has been of accelerating interest to ecologists since the writing of the modules. See Angilletta (2009, Thermal Adaptation) for a summary.] The material within is meant to complement these standard treatments. Even though all the examples are of animals, one can also think of plants as overstimulating. Plants modify leaf shape, position, and surface characteristics as well as daily and seasonal photosynthetic periods. The last part of the module develops the concept of the operative environmental temperature, which we will see is a useful way to integrate the effects of all heat transfer processes.