This book constitutes the refereed proceedings of the Third International Conference on Metalevel Architectures and Separation of Crosscutting Concerns, REFLECTION 2001, held in Kyoto, Japan in September 2001. The revised eleven long papers, seven short papers, and eight posters presented were carefully reviewed and selected from 44 submissions. The book offers topical sections on reflection and SOC in Java, software adaptation using reflection and SOC techniques, reflective middleware for distributed mobile applications, testing and verification, foundations of reflection and SOC, and software methodologies for SOC.