This book constitutes the proceedings of the 21st International Conference on Developments in Language Theory, DLT 2017, held in Liège, Belgium, in August 2017. The 24 full papers and 6 (abstract of) invited papers were carefully reviewed and selected from 47 submissions. The papers cover the following topics and areas: combinatorial and algebraic properties of words and languages; grammars acceptors and transducers for strings, trees, graphics, arrays; algebraic theories for automata and languages; codes; efficient text algorithms; symbolic dynamics; decision problems; relationships to complexity theory and logic; picture description and analysis, polyominoes and bidimensional patterns; cryptography; concurrency; cellular automata; bio-inspired computing; quantum computing.