A substitute decision-maker (SDM) is someone who is chosen to make medical decisions for a person should a severe illness or injury occur that makes communication impossible. The document that grants this decision-making power to the person selected is called a representation agreement, proxy directive, or other name depending on the province or territory.

An SDM may also be called a health care agent, proxy, surrogate, or an attorney-in-fact.

Although laws vary by province or territory, a substitute decision-maker can usually make medical treatment decisions (if health professionals agree with these decisions) at the end of life or anytime a person is not able to communicate. As soon as a person selects an SDM, it is important to thoroughly discuss and document medical care preferences, such as when to continue or abandon life-support measures.