The growth in wireless technology and the proliferation of mobile devices leads towards the new paradigm called Mobile Ad hoc NETwork (MANET). MANET is a self - organizing and self-administered network of wireless mobile nodes. MANET communicates on the basis of mutual trust. AODV protocol is the mainly used protocol in MANET. However, due to its openness, it suffers from various security breaches. Black hole attack and gray hole attack are popular attacks launched o AODV. In this paper, the authors have presented a wide survey of mitigation techniques to the black hole and gray hole attack.