The purpose of acute inpatient psychiatric care, and nurses’ role within that, are in need of clarification and restatement in order to provide a framework for practice, education, research and development. Inpatient psychiatry has suffered from a paucity of research in recent years. In addition, being a complex system, involving multiple professions with differing ideologies, means that widely accepted succinct descriptions of its purpose are hard to achieve. Yet such a framework is essential to support positive attitudes to patients and for good staff-management relationships. Using an oblique strategy, this paper defines the function of acute inpatient psychiatry, and the role of psychiatric nurses, via a structured examination of the literature on reasons for admission to acute inpatient psychiatric wards. Seven such reasons were discovered and are described: dangerousness, assessment, medical treatment, severe mental disorder, self-care deficits, respite for carers, and respite for the patient. Acute inpatient psychiatric nurses are therefore: providing safety for the patient and others; collecting and communicating information about patients, giving and monitoring treatment; tolerating and managing disturbed behaviour; providing personal care; and managing an environment where patients can comfortably stay. The implications for psychiatric nursing are discussed.