This chapter explores the multidimensional nature, etiology, epidemiology, and behavioral dynamics of anger and aggression, particularly in individuals with serious mental illness (SMI). Anger is conceptualized as a motivational state linked to goal obstruction, with adaptive and maladaptive expressions shaped by environmental contingencies, neurophysiological responses, and social learning. Aggression, distinct from anger, encompasses behaviors intended to harm and includes verbal, physical, and passive forms. Epidemiological data highlight increased aggression risk among individuals with co-occurring SMI and substance use problems, contributing to stigma and adverse outcomes. Dimensional models of anger, such as anger-arousal, rumination, frustration-discomfort, regulation, and socially constituted anger, are reviewed alongside psychometric limitations in existing assessment tools. Behavioral strategies emphasize stimulus control, collateral observation, and functional analysis of antecedents and consequences. The need for nuanced, multidimensional assessment, and intervention approaches that differentiate normative emotional responses from clinically significant dysfunction is highlighted.