 The dead are the major sources of organs for transplantation. For a long time deceased donors came from those declared brain dead, that is, those who have irreversibly lost their brain function. In recent years, however, many donors have come from those who have died in the sense of circulatory death. Both donation after brain death and donation after circulatory death invite the important philosophical—not just medical—question “what is it to be dead”? (See the entry on  the definition of death.)