This book constitutes the thoroughly revised selected papers from the 17th International Symposium, FACS 2021, which was hel virtually in October 2021. The 7 full papers and 1 short contribution were carefully reviewed and selected from 16 submissions and are presented in the volume together with 1 invited paper. FACS 2021 is concerned with how formal methods can be applied to component-based software and system development. The book is subdivided into two blocks: Modelling & Composition and Verification. Chapter "A Linear Parallel Algorithm to Compute Bisimulation and Relational Coarsest Partitions" is available open access under a Creative Commons Attribution 4.0 International License via link.springer.com.