Following Sen, social choice theorists often formulate rights in terms of relationships between individuals' preferences and social preferences. An alternative "procedural" formulation treats rights as properties of game forms. This paper reviews the debate between the proponents of these two approaches, focusing in particular on Sen's claim that the procedural approach is inflexible in its refusal to make trade-offs between rights violations. It looks at different answers to the question, "Why do rights matter?" It argues that, if a contractarian answer is given, there are good reasons not to make trade-offs.