A diagnostic assessment is a focused assessment of the client and the current and developmental context influencing the client. It is an essential competency in mental health practice. This chapter begins with a description of the diagnostic assessment. Then it discusses two different conceptual maps or ways in which the therapist “thinks” about the diagnostic assessment process. The second of the two ways, the integrative diagnostic assessment, is the one advocated in this chapter and is a key clinical competency in developing an effective case conceptualization. Next, the chapter outlines a “map” for performing an integrative diagnostic assessment. This procedure is illustrated with a transcription of an actual diagnostic interview.