Of the two major ethical issues surrounding organ allocation-determining criteria for expanding the size of the organ pool and determining criteria for allocation itself-I focus on the issue of allocation, and begin by assuming that there are five main criteria for use in deciding who gets a donor organ: age, medical benefit, merit, ability to pay, and geographical residence. I discuss each of these in turn, eliminating age because it fails to indicate the overall status of a patient's health; eliminating merit because physicians have neither the time nor the ability to act as judges; choosing medical benefit as the best criterion because it is fairest and does not call for such judgements; and leaving open considerations of ability to pay and geographical residence, for application in the event the issue cannot be decided on the ground of medical benefit alone. These criteria, I conclude, are best treated as guidelines, and not as rules.