The two-volume set, LNCS 9326 and LNCS 9327 constitutes the refereed proceedings of the 20th European Symposium on Research in Computer Security, ESORICS 2015, held in Vienna, Austria, in September 2015. The 59 revised full papers presented were carefully reviewed and selected from 298 submissions. The papers address issues such as networks and Web security; system security; crypto application and attacks; risk analysis; privacy; cloud security; protocols and attribute-based encryption; code analysis and side-channels; detection and monitoring; authentication; policies; and applied security.