This chapter explains a common theme running through the application of the law and ethics to medicine. The Mental Health Act (MHA) 1983 sets out the guidelines concerning the treatment. The MHA defines four categories of mental disorder such as mental illness, mental impairment, severe mental impairment and psychopathic disorder. Patients with mental disorder fall into two categories for the purposes of treatment depending on how they came to be in hospital: voluntary or informal admission and compulsory admission. Patients who understand that they have some form of mental disorder and are willing to comply with a treatment regime are considered to be informal patients. Compulsory admission procedures are used when there is sufficient concern about a patient's condition to warrant their admission into hospital for their own safety, health and well-being and that of those around them for the purposes of assessment or treatment.