The determination and pronouncement of when a person has died has medical, ethical, and legal implications. Laws vary from state to state, and death is usually defined in a state’s statutes.

Definitions and terminology may vary from state to state but generally a person is dead when, according to ordinary standards of medical practice, there is irreversible cessation of the person's spontaneous respiratory and circulatory functions. If artificial means of support preclude a determination that a person's spontaneous respiratory and circulatory functions have ceased, the person is dead when, in the announced opinion of a physician, according to ordinary standards of medical practice, there is irreversible cessation of all spontaneous brain function.

Death occurs when the relevant functions cease. But a formal pronouncement of death is not a legal determination of the cause, manner, or time of death.

In Montana, the determination of death is governed by state statutes which align with the general medical and legal standards for defining death. According to Montana Code Annotated 2019, Section 50-20-101, an individual is considered dead if they have sustained either an irreversible cessation of circulatory and respiratory functions, or an irreversible cessation of all functions of the entire brain, including the brain stem. The determination of death must be made in accordance with accepted medical standards. The pronouncement of death itself is a medical determination that confirms that death has occurred, but it does not establish the legal cause, manner, or exact time of death. These aspects are typically determined through a death investigation, which may involve medical examiners or coroners, especially in cases where the death is sudden, unexpected, or involves legal matters.