Abstract This article sets out basic legal principles underlying the care and treatment of mentally ill clients. Legal considerations are important for nurses, because it is now felt that the care of the mentally ill is as much a legal matter as a medical one. The frequent ordering of compulsory care, detention and administration of medication takes place in a climate, of ever-increasing emphasis on autonomy, and has resulted in the need for nurses to ensure that they do not violate clients' human and legal rights. Legislation is concerned not primarily with the finer points of what mental illness is – that is left to the medical profession – but with criteria which warrant the detention and treatment of a person against their will. The article points to international documents which set out principles for the care of the mentally ill, as well as both Commonwealth, state and territory legislation. It summarises the main provisions of these in relation to involuntary detention and treatment, as well as electroconvulsive therapy and psychosurgery. Nurses who care for mentally ill clients need, therefore, to understand in some detail the legislation that applies to them in their professional capacity, as well as the human rights it seeks to protect.