Vehicular ad hoc networks are emerging as an effective technology for providing a wide range of safety applications to by-vehicle passengers. Ensuring secure operation is one of the prerequisites for deploying reliable VANETs. In this article we argue that public key infrastructure is the most viable mechanism for securing VANETs as it can meet most VANET security requirements. However, PKI cannot provide certain security requirements such as location privacy, efficient authentication, and distributed and fair revocation. To complement the security services provided by PKI, we introduce complementary security mechanisms that can meet the aforementioned security requirements. Since denial of service attacks have severe consequences on network availability, which is one of the VANET security requirements, we propose a mechanism for mitigating the effect of DoS attacks in VANETs. Simulation results show that the complementary mechanisms together with PKI can efficiently secure VANETs.