A living will—also known as an Advance Health Care Directive—is a document in which the declarant or principal (person making the living will) specifies what kind of medical treatment the declarant does and does not want if the declarant has a medical emergency and is unable to communicate those wishes. A living will may direct health care providers to administer, withhold, or withdraw life-sustaining treatments if the declarant is in a terminal or irreversible condition.

Laws and terminology for documents related to living wills, Advance Health Care Directives, Do Not Resuscitate orders (DNRs), and other health care documents vary from state to state. These laws are generally located in a state’s statutes—often in the probate code or estates code.

In Maryland, a living will is known as an Advance Directive. It is a legal document that allows an individual to state their wishes regarding medical treatment in the event that they are unable to communicate due to a medical emergency. The Maryland Health Care Decisions Act governs the use of Advance Directives in the state. An Advance Directive can include instructions on the use of life-sustaining procedures if the individual is in a terminal condition, an end-stage condition, or in a persistent vegetative state. It can also appoint a health care agent to make decisions on behalf of the individual when they are unable to do so. Maryland law requires that the Advance Directive be in writing, signed by the declarant (or by another at the declarant's direction), and witnessed by two individuals. The state provides a statutory form, but individuals may also use other forms that meet the state's requirements. Do Not Resuscitate (DNR) orders are separate from Advance Directives and must be issued by a physician, indicating that in the event of a cardiac or respiratory arrest, resuscitation should not be attempted.