Abstract: Conference on Computer and Communications Security Vijay Atluri and Michael Backes and David Basin and Michael Waidner This volume contains the proceedings of the Second ACM Workshop on Formal Methods in Security Engineering (FMSE 2004) held in Washington D.C., October 29th, in conjunction with the 11th ACM Conference on Computer and Communications Security.The purpose of FMSE is to bring together researchers and practitioners from both the security and the software engineering communities, from academia and industry, who are working on applying formal methods to designing and validating large-scale security-critical systems. The scope of the workshop covers security and formal-methods related aspects of: security specification techniques, formal trust models, combination of formal techniques with semi-formal techniques like UML, formal analyses of specific security properties relevant to software development, security-preserving composition and refinement of processes, faithful abstractions of cryptographic primitives and protocols in process abstractions, integration of formal security specifications, as well as refinement and validation techniques in development methods and tools.The paper selection process was very competitive this year. The call for papers attracted 25 submissions from Asia, Canada, Europe, Africa, and the United States. The program committee accepted 9 papers for presentation at the workshop, which means that many high-quality papers had to be rejected. In addition, the program included an invited talk on &Suml;ecurity Analysis of Network Protocols" by John C. Mitchell as well as an invited talk on &Muml;odel-driven development of Security Components" by Prem Devanbu.