This chapter provides an overview of several mental illnesses, including schizophrenia, delusional disorder, bipolar disorder, post-traumatic stress disorder, and substance use disorders. Accurate knowledge of the diagnostic information allows justice system practitioners to become informed of the symptomology and better distinguish the different disorders. Because those with mental illnesses and substance use disorders interact with the criminal justice system at high rates and are overrepresented in correctional institution populations, understanding the characteristics can assist practitioners in better communicating and working more effectively with these populations. This chapter concludes by addressing and clarifying the common misconceptions of those living with mental illnesses and substance use disorders.