Rational individuals have their own purposes. They can pursue these purposes in many different ways. Quite often, individuals pursue their plans by exchanging with others whatever goods they happen to have, that is, by concluding contracts with others. The institution of contract, then, is a tool that individuals can use to make of their lives whatever they see fi t; it is a tool that they can use to exercise their moral power to set and pursue their own conception of the good. Now, in this book, I address the following question: if, as John Rawls famously suggests, justice is the fi rst virtue of social institutions, how are we to understand the institution of contract law? I argue that justice requires that we understand contract rules in relation to the idea of fair terms of interaction. The underlying idea is that of reciprocity, the idea that individuals should not set the terms of their interactions with others unilaterally.