By determining the microclimates that an animal experiences, habitats influence an animal's physiological capacities and ultimately its demographic and ecological performance. As a result, the ecology of organisms-especially of ectotherms-can be profoundly affected by the physiological consequences of habitat selection. Early ecologists such as Shelford and Chapman appreciated these issues, but most later ones tended to ignore physiology and instead focused on biotic interactions (e.g., competition). Recent technical and conceptual developments are now fostering a reintroduction of physiology into ecology. For issues relevant to thermal physiology, three steps are involved. First, the microclimates available in a habitat must be mapped. For ectotherms, this involves determining the operative environmental temperatures (Te)-that is, the potential body temperatures available in a habitat. Biophysical techniques can now generate Te maps with considerable accuracy. Second, the physiological effects of body temperature must be quantified. This requires laboratory studies of the effect of temperature on key performance traits. Third, the physiological suitability of habitats can be predicted by integrating the above environmental and physiological data. Analyses of the physiological conse- quences of habitat selection are exemplified in several case studies, and the importance of considering food and other factors in the analyses is stressed. An extension to endotherms is briefly discussed. The study of how and why organisms select particular habitats has long been central to ecology. In fact, the early literature of ecology was often dominated by discussions of habitat associations. Of special interest in the early days of ecology was the concept of limiting factors: which physical factors (e.g., tempera- ture, pH, salinity) limit the occurrence of organisms to particular habitats. Not surprisingly, early discussions often focused on interactions between physiology and the physical environment. Indeed, for many early ecologists, ecology and physiology were more or less synonomous concepts. Shelford, for example, de- fined ecology as "that branch of general physiology which deals with the organism as a whole . . and which also considers the organism with particular reference to its usual environment" (1913, p. 1). Similarly, Chapman argued: "The inter- relations of organisms in nature may be considered under the subject matter of ecology which is closely related to physiology, and in the minds of some biologists should be a subdivision of it" (1931, p. 3). Clearly, physiology was central to ecology at this time. In fact, physiological issues dominated nearly the first half of Chapman's (1931) book (Animal Ecology). Beginning with Elton (1927), however, ecological studies began to shift their focus from analyses of physiologically mediated interactions between organisms and their physical environments to analyses of interactions between individuals or between species, in other words, to analyses of population and community