This book presents an exploration of the idea of the common or social good, extended so that alternatives with different populations can be ranked. Basing rankings on the well-being, broadly conceived, of those who are alive (or ever lived), the axiomatic method is employed. Topics investigated include the measurement of individual well-being, social attitudes toward inequality of well-being, the main classes of population principles, principles that provide incomplete rankings or rank uncertain alternatives, best choices from feasible sets, and applications.