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 Illinois, the determination of death is governed by the Illinois Compiled Statutes, specifically the Illinois Uniform Determination of Death Act. According to this Act, an individual is considered legally dead when there is an irreversible cessation of circulatory and respiratory functions, or when there is an irreversible cessation of all functions of the entire brain, including the brain stem, in accordance with accepted medical standards. The pronouncement of death must be made by a licensed physician. It is important to note that the use of artificial life support may complicate the determination of death, and the law takes this into account by focusing on the irreversible cessation of brain function as an alternative criterion. The formal declaration of death itself does not establish the legal cause, manner, or exact time of death; these are separate determinations that may be made later, often as part of a death investigation or autopsy process.