Diagnostic assessment is the process of appraising a patient's condition. It involves effectively engaging the patient in order to obtain accurate information relevant to understanding health problems (mental and general medical disorders), their context (psychosocial and environmental problems) and their impact on adaptive functioning and participation in society (disablements). A comprehensive diagnostic formulation represents a summary of the clinician's judgement about the overall condition of the patient, obtained as much as possible with the latter's collaboration. The main purpose of diagnosis is to serve as the basis for clinical care. Further objectives include to communicate concisely and reliably information on health problems, to understand their biopsychosocial pathogenesis and the interaction of internal and contextual factors, to enhance training and research, and – last but not least – to inform a collaborative process of care aimed at the restoration and promotion of health, functioning and quality of life (Fig. 1.1).