This chapter offers a general introduction into the topic of commutative justice. Commutative justice is that kind of particular justice which has market exchanges as its subject – and a liberal theory of commutative justice, as developed in the book, answers the question “What is a just exchange?” from a liberal background. Furthermore, this chapter presents the main challenge which drives the book. This challenge is that, in light of collectively caused problems on a global level (global warming, global economic injustice, market imperialism, limits to growth), it might be insufficient to consider a market exchange just as soon as it is non-coercive, non-deceptive, non-exploitative, and so on. It seems like in determining whether an exchange is just, we cannot exclusively focus on the act of exchanging itself. We may also need to take into account the combined consequences of millions of market exchanges to reach a final judgment about whether some individual exchange is just. The chapter ends with outlining the plan for the book.