J. Rawls is considered as the most important philosopher of ethics and politics of the XX century. This article presents his fundamental ideas. His theory of justice as fairness aims to be the basis of a well ordered society, that makes possible the allocation of rights and obligations to the basic institutions, and the allows for the persistence of a stable and just society over time, although its citizens are divided by reasonable but incompatible doctrines. Equally, it must be the basis for the rules applicable to the international relationships between liberal and decent towns.