Among civilian communication systems, vehicular networks emerge as one of the most convincing and yet most challenging instantiations of the mobile ad hoc networking technology. Towards the deployment of vehicular communication systems, security and privacy are critical factors and significant challenges to be met. Thanks to the substantial research efforts carried out by the community so far, we make the following contributions in this paper: we outline security requirements for vehicular communication systems, we provide models for the system and the communication, as well as models for the adversaries, and propose a set of design principles for future security and privacy solutions for vehicular communication systems.