This book constitutes the refereed proceedings of the 12 th International Symposium on Privacy Enhancing Technologies, PET 2012, held in Vigo, Spain, in July 2012. The 16 full papers presented were carefully selected from 72 submissions. Topics addressed include anonymization of statistics, content, and traffic, network traffic analysis, censorship-resistant systems, user profiling, training users in privacy risk management, and privacy of internet and cloud-bases services. A further highlight is the HotPETS session, designed as a venue to present existing but still preliminary and evolving ideas.