The behavioral complexities associated with individuals diagnosed with serious mental illness involve both maladaptive behaviors and deficits in expected functional behaviors. This includes a delineation of positive symptoms, such as hallucinations, delusions, disorganized speech, aggression, and affective lability, as well as negative symptoms, including avolition, anhedonia, alogia, and impairments in social and self-care functioning. An analysis examines how these behaviors differ across various dimensions such as frequency, intensity, context, and consequence, frequently leading to stigma, reduced daily functioning, and lower quality of life. It addresses the complexities inherent in evaluating both covert and overt behaviors, the effectiveness of interventions targeting negative symptoms, and the ongoing functional limitations that may persist even after measurable improvements. It emphasizes the importance of individualized, multimodal assessment and behavior analytic intervention strategies to account for the heterogeneity and multidimensionality of behavioral deficits in individuals living with serious mental illness, and advocates for recovery-oriented care focused on restoring adaptive functioning and independence.