The purpose of this review is to summarize certain approaches toward investigating animal behavior proposed by physiologists, physicists, ecologists and others. The failure of classic reflex theory in the analysis of complex forms of animal behavior has been demonstrated. The peculiarities of the functional system theory, which is one of the most popular theories in neurophysiological circles of the USSR, have been described. The application of the functional system theory to an investigation of feeding behavior has been shown. The strong and weak points of the functional system theory have been indicated, and the place of this theory among other system theories proposed for an analysis of behavior has been discussed.