Mobile Ad Hoc Networks (MANETs) are a spontaneous network of mobile devices that do not rely on any kind of fixed infrastructure. In these networks, all the network operations are carried out by nodes themselves. The self-organizing nature of MANETs makes them suitable for many applications and hence, considerable effort has been put into securing this type of networks. Secure communication in a network is determined by the reliability of the key management scheme, which is responsible for generating, distributing and maintaining encryption/decryption keys among the nodes. In this paper we investigate key management schemes for MANETs. We give an overview of available key management schemes for symmetric key, asymmetric key, group key and hybrid key cryptography