Optimal foraging theory (OFT) is one of the few areas of study in behavior and ecology in which mathematical models derived from first principles have been seriously tested in the laboratory and field. The balance between theory and data has remained good, unlike, for example, the field of community ecology in the 1960s and 1970s, where arcane models of baroque complexity were generally matched only with qualitative and unconvincing tests. Part of the success of OFT lies in the fact that although ecological in origin, the models have been tested with both ecological methods and the methods developed by ethologists and comparative psychologists (Pyke et al. 1977; Krebs 1978, Staddon 1980; Hughes and Townsend 1981; Kamil and Yoerg 1982). In 1966, the first two papers published on OFT (MacArthur and Pianka 1966; Emlen 1966) amounted to 0.5% of the articles in American Naturalist, Ecology, Journal of Animal Ecology, and Animal Behavior. The proportion of papers on OFT in just these journals had quadrupled to 2% by 1974 and to 8% in 1981. In this chapter, we will start with a brief general comment on optimal foraging theory, then we review the evidence relating to “classical” foraging models. This is followed by two more detailed discussions; the first considers the relationship between classical models and two more recent developments, models of “rules of thumb” and stochastic models, and the second looks at some implications of the traditional models for population interactions of predators and prey.