Agitation is a nonspecific constellation of relatively unrelated behaviors that can be seen in a number of different clinical conditions, usually presenting a fluctuating course. Multiple underlying pathophysiologic abnormalities are mediated by dysregulations of dopaminergic, serotonergic, noradrenergic, and GABAergic systems. Pathophysiologic mechanisms of agitation that operate in the different clinical disorders where agitation occurs are discussed. These pathophysiologic abnormalities are not associated with distinct clinical features. Although there may be a final common pathway, there is no unifying etiologic pathophysiology. The author suggests that the clinician address the underlying pathophysiology through a treatment intervention that addresses the overarching psychiatric disorder. Generally, agents that reduce dopaminergic or noradrenergic tone or increase serotonergic or GABAergic tone will attenuate agitation, often irrespective of etiology.