A comprehensive examination of the psychological and biological factors involved in exercise addiction is presented in an attempt to explain the varying position of individuals on the continuum of ability to commence and continue a regimen of physical activity. Physiological considerations in exercise tolerance and the feelings derived from exercising are discussed. The psycho-social perspective is examined with consideration given to a number of theoretical orientations in this area, including Freud's drive reduction theory, Petrie's augmentation-reduction model, Eysenck's research on extraversion-introversion and the opponent-process theory of motivation, theories primarily relating to the phenomenon of exercise addiction. Psycho-biological mechanisms are identified which encourage a high frequency of participation in physical acitivity such that participants are considered to be habituated. (Author/MB)