This book constitutes the refereed proceedings of the 12th International ACM SIGSOFT Symposium on Component-Based Software Engineering, CBSE 2009, held in East Stroudsburg, PA, USA in June 2009. The 16 revised full papers presented were carefully reviewed and selected from 43 submissions. The papers are organized in topical sections on component models: taxonomies and applications, component integration: patterns and profiling, communication and composition, extra-functional analysis, as well as components within the development life cycle.