A person’s decision-making capacity is pivotal when a doctor determines whether a person has the ability to make his or her own decisions, and thus it has major clinical, ethical, and legal significance, especially in Hong Kong, where the Mental Health Ordinance has introduced Guardianship provisions and provisions for the treatment of mentally incapacitated persons. This paper examines the legal concept of decision-making capacity and its clinical assessment within the local context. It is important for doctors to be aware of the legal mechanisms under which both urgent and non-urgent treatment can be given to mentally incapacitated persons, provided that the treatment is necessary and given in the best interests of the patient.