Today use of ad hoc networks is increased gradually in the people day-to-day life, and there are vulnerabilities in Mobile Ad hoc NETwork (MANET) due to their natural characteristics. There are several cryptographic mechanisms for MANET; one of them is identity based cryptosystem and key management for security and efficient key management. Identity based cryptography is found to make key management simple and reduce memory cost of conventional public key system. In this paper, the authors will study about existing identity based key management schemes, their pros and cons and comparing their main features. This paper will help users and researchers to build strong and robust key management scheme.