This book constitutes the proceedings of the 11th International Symposium on Frontiers of Combining Systems, FroCoS 2017, held in Brasília, Bazil, in September 2017.   The 17 papers presented in this volume were carefully reviewed and selected from 26 submissions. They were organized in topical sections named: description and temporal logics, decision procedures, decidability and verification, SAT, SMT and automated theorem proving, term rewriting, and properties and combinations of logics. The paper 'Subtropical Satisfiability' is open access under a CC BY 4.0 license via link.springer.com. .