This book constitutes the proceedings of the 21st International Conference on Descriptional Complexity of Format Systems, DCFS 2019, held in Košice, Slovakia, in July 2019.

The 18 full papers presented in this volume were carefully reviewed and selected from 25 submissions. The book also contains 4 invited talks. They deal with all aspects of descriptional complexity and costs of description of objects in various computational models, such as Turing machines, pushdown automata, finite automata, grammars, and others.