This book constitutes the proceedings of the 21th International Conference on Coordination Models and Languages, COORDINATION 2019, held in Kongens Lyngby, Denmark, in June 2019, as part of the 14th International Federated Conference on Distributed Computing Techniques, DisCoTec 2019. The 15 full papers included in this volume were carefully reviewed and selected from 25 submissions. The papers are organized in topical sections named: computational models; tools; exploring new frontiers; and coordination patterns.