Social workers know that an accurate diagnosis, when possible, is essential to figuring out the appropriate treatment. This chapter provides an overview of the various types of mental health disorders and their corresponding treatments. The lack of consensus and clear paths to treatment is a major impediment to advancing the field of mental healthcare. The classification of mental illnesses is important because it provides a common language and the ability to communicate in an organized way. The two most prominent classification systems for mental illness are the International Classification of Diseases and the Diagnostic and Statistical Manual of Mental Disorders (DSM). The chapter discusses major mental disorders such as schizophrenia, mood disorders, bipolar disorder, personality disorders, and suicidal behavior disorder. It reviews different types of inpatient and outpatient treatment such as cognitive behavioral therapy and dialectical behavior therapy. Finally the chapter discusses co-occurring disorders/dual diagnoses.