The Intrusion Detection System (IDS) protects the VANETs from security threats by continuously monitoring the system behavior. Based on the attack detection strategy, the IDS models are categorized into anomaly-based, signature-based, and specification-based. The IDS architectures are also classified into three categories that are standalone, cooperative and distributed, and hierarchical. After cryptography based solutions, the IDS models are referred to as the second level of a defense often used for VANET security.