Great advances have been made during the last 20 years in the development of structured and semi-structured interviews for use with psychiatric patients. However, in the field of child and adolescent psychiatry there have been weaknesses in the specification and definition of both symptoms and the psychosocial impairments resulting from psychiatric disorder. Furthermore, most of the available interviews for use with children have been tied to a single diagnostic system (DSM-III, DSM-III-R, or ICD-9). This has meant that symptom coverage has been limited and nosological comparisons have been inhibited. The Child and Adolescent Psychiatric Assessment (CAPA) represents an attempt to remedy some of these shortcomings. This paper outlines the principles adopted in the CAPA to improve the standardization, reliability and meaningfulness of symptom and diagnostic ratings. The CAPA is an interviewer-based diagnostic interview with versions for use with children and their parents, focused on symptoms occurring during the preceding 3 month period, adapted for assessments in both clinical and epidemiological research.