John Rawls' theory of justice as fairness describes a society of free citizens with equal basic rights who work together cooperatively in an egalitarian economic system. Through its liberal political conception of justice, his theory provides a framework for the legitimate use of political power. However, legitimacy is only the minimum standard of moral acceptance; a political order can therefore be legitimate without being just per se. Justice, on the other hand, sets the maximum standard: the arrangement of social institutions that is morally best.

Rawls constructs justice as fairness in the context of specific interpretations of the ideas that citizens are free and equal and that society itself should be fair. Social cooperation in some form is necessary for citizens to live a decent life. However, they are not indifferent to how the benefits and burdens of cooperation are distributed among them. Rawls's principles of justice as fairness express the central liberal ideas that cooperation should be fair to all citizens who are considered free and equal.

The characteristic interpretation that Rawls gives to these concepts can be seen as a combination of a negative and a positive thesis. Rawls's negative thesis is based on the idea that citizens do not deserve to be born into a rich or poor family, to be naturally more or less gifted than others, to be born female or male, to belong to a particular race, and so on. Since these characteristics of persons are morally arbitrary in this sense, citizens are not entitled to more benefits of social cooperation for this reason alone. For example, the fact that a citizen was born rich, white and male is not in itself a reason to favor that citizen by social institutions. However, this negative thesis says nothing about how social goods should be distributed; it merely clarifies the preconditions. Rawls' positive distribution theory is reciprocity based on equality. All social goods are to be distributed equally, unless an unequal distribution would be beneficial to all. The guiding principle is that since citizens are fundamentally equal, considerations of justice should be based on the assumption that cooperatively produced goods should be distributed equally. Justice then requires that all inequalities must benefit all citizens, especially those who have the least of them. Equality is the foundation; from there, any inequality must improve the situation of all, especially the worst off. These strict requirements of equality and mutual advantage are hallmarks of Rawls' theory of justice.