A MANET is a type of ad hoc network that can change locations and configure itself on the fly. Since the nodes are mobile, the network topology may change rapidly and unpredictably over time. The network is decentralized, where all network activity including discovering the topology and delivering messages must be executed by the nodes themselves, i.e., routing functionality will be incorporated into mobile nodes. The open medium and wide distribution of nodes makes MANET vulnerable to malicious attacks. To address the security, it is essential to develop an Intrusion Detection System (IDS) specially designed for MANET which can detect malicious attacks before they do any significant damage to the network.