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 Massachusetts, living wills are not explicitly recognized by statute, but the state does recognize health care proxies, which are similar to Advance Health Care Directives. Under Massachusetts law, specifically Massachusetts General Laws Chapter 201D, an individual may appoint a health care agent through a health care proxy to make medical decisions on their behalf if they become incapacitated and are unable to make or communicate their own decisions. The health care agent is authorized to make decisions that are consistent with the known wishes of the principal, including decisions about life-sustaining treatment. While a living will is not legally binding in Massachusetts, it can still serve as important evidence of the individual's wishes regarding medical treatment and end-of-life care. Additionally, Massachusetts recognizes Do Not Resuscitate (DNR) orders, which are separate documents that instruct health care providers not to perform CPR if a patient's breathing or heartbeat stops.