Mobile Ad Hoc networks (MANETs) are susceptible to having their effective operation compromised by a variety of security attacks. For example, misbehaving nodes can cause general network disruption by not forwarding packets on behalf of other nodes in the network. Nodes may misbehave either because they are malicious and deliberately wish to disrupt the network, or because they are selfish and wish to conserve their own limited resources such as power, or for other reasons. In this paper, we present a mechanism capable of detecting and accusing nodes that exhibit packet forwarding misbehavior. Our evaluation results demonstrate that our algorithm effectively detects and accuses nodes that drop a significant fraction of packets.