Aggregation is one of the fundamental features of utilitarianism and other forms of axiological theories, permitting the trade-off of morally relevant factors between different individuals. It is also a feature that critics of utilitarianism condemn. Yet the basic features of aggregation are not understood properly. This book elucidates the theoretical structure and scope of interpersonal and intra-personal aggregation in ethics and defends a form of aggregation, formal aggregation, as distinguished from substantive aggregation of utilitarianism.