Wireless sensor networks are exposed to different types of security threats that can reduce the performance of the whole network; that might result in serious problems like denial of service attacks, routing attacks, Sybil attack, etc. Defensive mechanisms like Key management protocols, authentication protocols and secure routing cannot provide security to WSNs for these types of attacks. Intrusion detection system is a solution to this problem. It analyses the network by collecting sufficient amount of data and detects anomalous behavior of sensor node.