Vehicular Adhoc Network is a special form of the Mobile Adhoc Network which has received remarkable attention with the growing era. It is capable of providing data communication between the mobile vehicles. It focuses on safety related applications and Internet accessing related applications. The basic model of the VANETs consists of On Board Units (OBUs) and Road Side Units (RSUs). These units communicate through an open wireless medium, creating a threat to the network. As the malicious node can attack the privacy information such as the user's identity, tracing preferences etc., if the node is not properly protected. Therefore, there is a need of security requirements to secure it. The paper describes various requirements, characteristics, challenging issues and techniques of security in VANETs. In this paper a theoretical analysis of different security techniques of Vehicular Adhoc Networks has been done which compare different schemes at different levels like hardware, authentication, privacy and certification techniques etc.