This book constitutes the revised selected papers from the 12th International Conference on Formal Aspects of Component Software, FACS 2015, held in Niterói, Brazil, in October 2015. The 15 full papers and 2 invited papers presented in this volume were carefully reviewed and selected from 33 submissions. They are organized in topical sections, namely quality of service to withstand faults, component-based software development through research on mathematical models for components, composition and adaptation; rigorous approaches to verification, deployment, testing, and certification.