To enable proper functioning of wireless ad hoc networks, all nodes in the network are expected to cooperate in forwarding each other?s packet. But relaying other nodes? packets involves spending energy without getting any immediate revenue. Hence, due to the constraints on available energy the nodes usually have in such networks, from an individual node?s point of view, the best response is always not to cooperate. We model this problem as a repeated game and show analytically that given a suitable punishment mechanism, individual nodes can be deterred from their selfish behavior, and cooperation can emerge as the best response. We also show that generosity by other nodes cannot be part of a credible deterrent mechanism.