The chapter explores the complex and multidimensional nature of suicide and self-harming behaviors in individuals living with serious mental illness (SMI). It defines suicidal behaviors as a continuum of self-directed actions with intent to die, and distinguishes them from non-suicidal self-injury (NSSIB), which serves affect regulation without lethal intent. Epidemiological data reveal elevated suicide rates globally, with disproportionate impact on youth, males, and marginalized populations. Etiological factors include mental health conditions like major depression, bipolar, schizophrenia, and personality-related mental health problems, trauma, stigma, substance use, and neurobiological vulnerabilities such as impulsivity and serotonergic dysregulation. Behavioral strategies emphasize the role of emotional dysregulation, interpersonal conflict, and method lethality in shaping risk. Accurate differentiation and early intervention are critical to reducing morbidity and mortality in SMI populations by using dimensional assessment models incorporating psychological, biological, and social domains. There is a need for standardized definitions, culturally sensitive interventions, and integrated prevention efforts targeting high-risk groups.