The forms and behavioral correlates of yawning are described, and the phylogenetic and ontogenetic aspects of the act are examined with particular attention to its possible functions. Much evidence supports the view that yawning is an important mediator of behavioral arousal levels, a view that is further strengthened by a review of endocrine, neurotransmitter, and pharmacological mechanisms of yawning: A major function of yawning appears to involve maintenance or increase of arousal when environments provide relatively little stimulation.