The article defines the terms "requirement", "need" and "need of treatment", and explains their significance in the field of psychiatric care. The fundamentals of assessing the requirement are discussed, as well as the associated problems: recording and registration of patients in need of treatment, their readiness to undergo such treatment, and possible alternatives of hospitalisation. For establishing criteria required to determine the need treatment symptoms from two semi-standardised psychiatric interviews are examined for suitability (Present State Examination and Goldberg Interview); likewise, the severity of the disease, the stress from which the patient suffers, and the impairment of social relations, are also examined for suitability. It is suggested to take these criteria as basis and to operationalise the need of treatment on these lines. Relevant studies to assess the usefulness of these criteria are under way in two research projects of the Special Research Division 166 "Psychiatric Epidemiology".