In recent years, the use of mobile ad hoc networks (MANETs) has been widespread in many applications and security has become one of the major concerns in MANETs. An Ad-hoc network is one that is an autonomous, self-configuring network made up of mobile nodes connected via wireless links. The mobile or portable devices are free to move at any rate/direction and are part of the network only when they are within range and due to these unique characteristics of MANETs, prevention methods alone are not sufficient to make them secure. Hence there is a strong need of Intrusion detection systems as a second line of defense for securing MANET. Most intrusion detection systems for mobile ad hoc networks are focusing on either routing protocols or its efficiency, but it fails to address the security issues. In this paper, we classify and discuss the various attacks, techniques used for intrusion detection are discussed and general issues that should be considered while implementing in intrusion detection system in Mobile Ad hoc Network (MANET) to provide high performance to the network.