The article introduces John Rawls' book A Theory of Justice, which is perhaps that contribution in political philosophy that attracted the most attention in the twentieth century. After a brief biographical introduction, Part 2 presents the allocation principles John Rawls advocated. Part 3 presents Rawls' conception of society and the individual, as an introduction to the rest of the argument presented in Part 4. Section 5 takes up his theory of justification, and Part 6 points to three areas where the more recent book Justice as Fairness differs somewhat from A Theory of Justice. Section 7 presents some of the criticisms that have been raised, and Section 8 points to some lasting contributions of Rawls' theory.