The main assumption of ad hoc routing protocols is that all participating nodes do so in good faith without disrupting the operation of the protocol. However due to the absence of properly protected media and well trusted infrastructures, and the reliance on unknown third parties for data forwarding, MANETs are intrinsically vulnerable to various attacks which can compromise the confidentiality, integrity and authenticity of the exchanged information. The security issue of MANETs is even more challenging because of the involvement of multiple senders and multiple receivers. Achieving trustworthy and secure and reliable communication is a major technical challenge in MANETs.