The degree to which organisms can exert control over events to which they are exposed has a strong impact on behavior and physiological functioning. Effects caused by the uncontrollability of events that are beyond the organism's control rather than by the events per se have been called learned helplessness effects. The present paper reviews such learned helplessness effects. At a behavioral level, uncontrollable aversive events result in associative, motivational, and emotional deficits. At a neurochemical level, uncontrollable but not controllable aversive events have been reported to lead to disturbances in cholinergic, noradrenergic, dopaminergic, serotonergic, and GABAergic systems. However, there are interpretive difficulties in this literature, and these are discussed. The controllability/uncontrollability of aversive events has a role in producing stress-induced analgesia and the activation of endogenous opiate systems. These relationships are reviewed. It is proposed that the learning that aversive events cannot be controlled activates an opiate system. The research reviewed is related to depression, and the general issue of animal models of depression is discussed. It is concluded that no experimental paradigm can be a model of depression in some general sense, but can only model a particular aspect. Learned helplessness may model "stress and coping".