This chapter describes the process of diagnosis, which is part of the assessment that takes place in the beginning of treatment. Diagnosis is a way to describe, in standardized form, the client's presenting symptoms, other behavioral health conditions, and related health concerns. The chapter explains the diagnostic manual that is most commonly used in behavioral health settings. It explores how to assign an accurate diagnosis and how to use the DSM-5. Many clinicians in training question the reasons for assigning a diagnosis to a client. The author's would use a different treatment approach with a client whose symptoms of lethargy and lack of motivation are part of a diagnosis of Major Depressive Disorder than one with the same symptoms whose diagnosis is Schizophrenia. Third parties such as public or private insurance or a governmental agency generally require a primary behavioral health diagnosis, and will not fund treatment without it.