Ad-hoc networks are emerging technology, due to their spontaneous nature, are frequently established insecure environments, which makes them vulnerable to attacks. These attacks are launched by participating malicious nodes against different network services. Ad hoc On-demand Distance Vector routing (AODV) is a broadly accepted network routing protocol for Mobile Ad hoc Network (MANET). Black hole attack is one of the severe security threats in ad-hoc networks which can be easily employed by exploiting vulnerability of on-demand routing protocols such as AODV. In this paper, a review on different existing techniques for detection of pooled or co-operated black hole attacks with their defects is presented.