Vehicular Ad hoc Networks (VANETs) are special kind of Mobile Ad hoc Networks (MANETs), where vehicles communicate with each other in ad hoc formation. VANETs consist of Vehicles and Road Side Units (RSUs) that assist in the network management. Vehicles communicate with each other and RSUs, with the aim to provide infotainment and safety services on road. Security is an important consideration in VANETs as safety of humans (passengers) is an important issue. Vehicular Public Key Infrastructure (VPKI) is an adapted form of Public Key Infrastructure (PKI) used to achieve the key management and security services in VANETs. Certificate generation and revocation is one of the primary functions of VPKI. Certificate revocation is used for revoking the malicious nodes and terminate their access rights to the network. In this paper we classify revocation schemes in a novel way into centralized and decentralized manners. This paper covers a survey of different certificate revocation schemes, and provides an overview of the research in the area of certificate revocation in VANETs.