Among classical Greek and Roman societies the signs of death were the absence of a heartbeat and breathing, and the onset of putrefaction. In medieval times a candle was held to the mouth - a flicker of the candle was shown as a sign of life.

However, these signs were rejected by anatomist Jacques-Benigne Winslow in 1740, who recommended that resuscitation should be attempted on seemingly lifeless patients by stimulating various parts of the body with the "juices of onions, garlic and horse-radish, . . . whips and nettles, ... and by hideous Shrieks and excessive Noises." Pins were also inserted under the toenails.

In 1742 John Bruhier documented fifty-two examples of supposed live burial, in his book Dissertation de l'incertitude des signes de la mort, This fed the public's fears of premature burial, and placed growing pressure on doctors to come up with more reliable 'signs of death' as a diagnostic tool. German doctors concluded that putrefaction was the only reliable indicator of death. A number of cultures include an interval between death and disposal of the body that allows time for putrefaction. For example, the leichenhäuser (corpse houses) of 19th century Germany provided a place where 'corpses' were kept under surveillance until putrefaction was apparent.

The 'Safety Coffin' highlights people's uncertainty about pinpointing the moment of death and their fear of being buried alive. The safety coffin provided a means for "deceased" to signal the world above for help and salvation.
 

More immediate and drastic techniques have also been employed to ensure that the dead were really dead. Some people requested in their wills, that their head be severed, or their heart be pierced prior to burial as a measure to ensure they were truly dead. The invention of the stethoscope in 1819 removed the need for these extreme measures.

But medical intervention has also increased uncertainty. The invention of the artificial respirator in the 1950s meant that the cells of the body could be kept alive in the absence of a natural heartbeat. By 1968 when the first heart transplant was performed, it was already clear that there needed to be a diagnosis for death that was not based on heartbeat. A committee based at the Harvard Medical School in the USA, came up with a diagnostic criteria for death called brain death criteria.
Brain death (irreversible cessation of all function of the brain) normally occurs after a stroke, or an impact that causes the brain to swell and push against the skull, preventing blood from flowing to the brain. In the absence of oxygenated blood, brain cells quickly die. The dead cells break down and liquefy. Brain death is quite different from reversible coma (unconsciousness) in which living brain cells remain.

A person can remain permanently unconscious with total or partial brain death. A person with death of only the upper brain (cerebral hemispheres) will not have consciousness, memory, knowledge or thought, but the living lower brain (brain stem) allows the heart to pump, the lungs to breathe and the body to function.

To be legally brain dead, all function of both the upper and lower brain must cease. Because the heart will fail on a brain-dead person, certification of death by brain-death criteria (instead of circulatory criteria) will only be needed when the dead person's body functions are being maintained by an artificial ventilator.

To establish that the brain is dead, certifying doctors must ascertain that:

• there is no evidence of brain function over a period of time
• the loss of function is not a result of drugs, hypothermia (low temperature), hypoglycaemia (low blood sugar) or hyponatraemia (low blood sodium)
• the person has sustained a brain injury sufficient to account for the irreversible loss of brain function. Often this is done by CT scan
• there are no reflex functions associated with coughing, gagging, eye movement, blinking, or dilation of the pupils
• the person makes no attempt to breathe when disconnected from the respirator for several minutes
• during the previous test, the carbon dioxide level of the blood has risen above the point at which breathing is normally stimulated
• These tests are frequently repeated after a further 24 hours as an assurance of irreversibility. A flat electroencephalogram, indicating an absence of brain activity is often used for verification.

Where the "dead" person's organs are available for donation, two doctors, neither of whom is caring for a potential organ recipient, must undertake the testing and certification.

If the person has received drugs that might invalidate the testing procedure, or if the head is so badly damaged that the tests cannot be performed, death can be determined by total lack of blood flow to the brain. This is done by inserting dyes (angiogram), or radioisotopes into the blood vessels supplying the brain blood vessels, to ascertain that they do not travel to the brain.