In recent years mobile ad hoc network has a great impact on wireless networks. In MANET, each node acts as a router to establish a route and transfer data by means of multiple hops. MANET are more vulnerable to security problem. When a node wants to transfer data to another node, packets are transferred through the intermediate nodes, thus, searching and establishing a route from a source node to a destination node is an important task in MANETs. Routing is an important component in MANET and it has several routing protocols. Ad hoc On-demand Distance Vector (AODV) is one of the most suitable routing protocol for the MANETs and it is more vulnerable to black hole attack by the malicious nodes. A malicious node that incorrectly sends the RREP (route reply) that it has a latest route with minimum hop count to destination and then it drops all the receiving packets. This is called as black hole attack. In the case of multiple malicious nodes that work together with cooperatively, the effect will be more. This type of attack is known as cooperative black hole attack. There are lots of efforts have been made to defend against black hole attack, but none of the solution looks most promising to defend against black hole attack. So in this paper, We have surveyed and compared the existing solutions to black hole attacks on AODV protocol.