World peace, social cohesion, and integral human development all depend to a considerable degree on the attainment of an appropriate balance of goods, which respects the individual person or group and the whole society or community. Pope John Paul II frequently proposes solidarity as the means by which this «common good» can be attained. But what is solidarity, and how does it achieve its ends? This work studies the development of the concept of solidarity in the philosophy of Karol Wojtyla as well as in his addresses and public writings as pope. The influences of Durkheim, Mounier, and Scheler, as well as the influence that earlier Catholic social teaching had on Wojtyla, are discussed in detail.