This book constitutes the refereed proceedings of the First International Symposium on Engineering Secure Software and Systems, ESSoS 2009, held in Leuven, Belgium, in February 2009. The 10 revised full papers presented together with 7 industry reports and ideas papers were carefully reviewed and selected from 57 submissions. The papers are organized in topical sections on policy verification and enforcement, model refinement and program transformation, secure system development, attack analysis and prevention, as well as testing and assurance.