The present document specifies a security architecture for Intelligent Transport System (ITS) communications. Based upon the security services defined in TS 102 731 [4]@ it identifies the functional entities required to support security in an ITS environment and the relationships that exist between the entities themselves and the elements of the ITS reference architecture defined in EN 302 665 [1]. The present document also identifies the roles and locations of a range of security services for the protection of transmitted information and the management of essential security parameters. These include identifier and certificate management@ PKI processes and interfaces as well as basic policies and guidelines for trust establishment.