Mobile Ad hoc networks are playing very important role in the present world. They are applied to several popular wireless technologies including cellular phone services, disaster relief, emergency services, battlefield scenarios, and other applications. MANETs are decentralized networks, and the network topology is unpredictably dynamic because of node mobility. As a result, mobile nodes in MANETs act as both hosts and routers since MANETs are decentralized; all mobile nodes need to discover the dynamic topology and deliver messages by themselves. MANETs rely on the cooperation of all mobile nodes in the network to ensure reliable routing services in the presence of dynamic topology caused by their mobility. The dynamic and cooperative nature of MANETs presents substantial challenges for network security. Therefore, sufficient protection should be provided to secure MANETs to guarantee the integrity of routing messages and availability of routing services. In other words, the goal of this dissertation is to examine how to secure the routing services of MANETs in order to provide reliable communication among nodes.