In mobile Delay Tolerant Networks (DTNs), messages are routed in a mobility-assisted manner. Therefore, the willingness of nodes to relay messages for other nodes plays a significant role in the routing process. Moreover, since the resources in mobile devices are generally limited, carriers of mobile devices may be unwilling to relay messages for other nodes in order to conserve their scarce resources. Furthermore, recent studies on the impact of selfish behavior in DTNs show that the performance metrics (i.e., delivery ratio, delivery overhead, and delivery latency) are severely influenced if a major portion of the nodes is selfish. A number of strategies have been proposed to prevent the impact of selfish behavior. It is therefore necessary to give an overview of the representative strategies.