This chapter examines the classification and conceptualization of serious mental illness, outlining its development from Emil Kraepelin’s early twentieth-century nosology to present-day interpretations. Kraepelin’s distinction between exogenous and endogenous disorders set the stage for modern frameworks, such as the DSM-V-TR and ICD-11, by introducing a systematic and clinical approach to symptom patterns and disease course. The discussion emphasizes the multidimensional aspects of mental health challenges, highlighting variations in symptoms, daily functioning, and response to interventions. Key topics include the prevalence and consequences of serious mental illness, such as increased rates of trauma exposure, substance use, and involvement with the criminal justice system among those affected. The concept of anosognosia, or lack of awareness of one’s illness, is identified as a critical obstacle to treatment engagement. The importance of individualized, evidence-based strategies is underscored, with a focus on improving adaptive functioning and quality of life for people living with serious mental illness.