Meaningful psychiatric psychodiagnostic work demands an understanding of the whole person. The psychologist using psychological tests can play a crucial role in helping treatment providers appreciate the complexity of the whole person. This chapter discusses psychodiagnostic work in the treatment of hospitalized psychiatric patients. The authors detail an approach to diagnosing and treating that is dynamic, collaborative, multidisciplinary, and person-centered, reflecting a longstanding testing tradition that began evolving at the Menninger Clinic in the 1940s. The chapter highlights the role of clinical psychologists on multidisciplinary teams and in the diagnostic process. The psychologist uses a battery of psychological tests to systematically investigate the nature and meaning of the patient’s psychological problems. Patients are enlisted as a partner in the testing process, and their active engagement is sought to reach an understanding of their difficulties and how treatment can be of help. Case examples illustrate how the psychologist engages with the test data, the patient and the inpatient treatment team to gather diagnostic information, establish emotional attunement, develop understanding of self and relationships, create a treatment focus, facilitate development of alliance and develop a road map for effective treatment.