The advance in wireless technologies makes now viable to start to develop ad hoc networks. However, without a mechanism that prevents misbehaviors, these networks could result easily unreliable. We develop a model, based on game theory, capable of formally explaining characteristics of ad hoc networks (as the nodes' selfishness or the network mobility). It also allows to formally study and analyze strategies for cooperation. As example, we describe a simple strategy that enforces packet forwarding among nodes.