Mobile Ad-hoc NETwork (MANET) is constructed from a collection of nodes that can move anywhere and anytime in different areas without any infrastructure that means MANET is infrastructure less. Each node works at the same time as router and host. Lack of a fixed infrastructure, wireless medium and dynamic topology makes MANET vulnerable to different kinds of attacks like gray hole and black hole. In this paper, the authors investigate different mechanisms that have designed to detect or prevent black or gray hole attacks in AODV protocol. They discuss about advantages and disadvantages of the different methods.