ABSTRACT In the Roman Catholic tradition the nature of the ordinary/extraordinary means distinction is best understood in light of its historical development. The moralist tradition that reared and nurtured this distinction implicitly developed a set of general criteria to distinguish the extraordinary from the ordinary. These criteria, conjoined with the context within which they were understood, can play an important role in refereeing the contemporary debate over the agressiveness of medical treatment and the extent of one's moral obligation.