This chapter examines the multifaceted impairments in social functioning among individuals living with serious mental illness (SMI), emphasizing dimensional, etiological, epidemiological, and behavioral frameworks. Social dysfunction is characterized by deficits in pragmatic communication, emotional expressiveness, and motivation-related behaviors such as avolition and asociality. Epidemiological data reveal high prevalence and persistence of negative symptoms, which are poorly responsive to pharmacological interventions and strongly predictive of impaired social outcomes. Etiological factors include neurocognitive deficits, social cognitive impairments (e.g., Theory of Mind, attributional style), and environmental deprivation. Behavioral strategies highlight the role of operant learning, motivational operations, and skill acquisition deficits, with emphasis on the need for context-sensitive interventions targeting both expressive and receptive social domains. Dimensional models offer superior predictive validity over categorical approaches, supporting tailored assessments and interventions. Integrated behavioral and neurocognitive strategies are needed to enhance social competence and real-world functioning in individuals with SMI.