This book constitutes the proceedings of the 20th International Conference on Developments in Language Theory, DLT 2016, held in Montreal, QC, Canada, in July 2016. The 32 full papers and 4 abstracts of invited papers presented were carefully reviewed and selected from 48 submissions. This volume presents current developments in formal languages and automata, especially from the following topics and areas: combinatorial and algebraic properties of words and languages; grammars, acceptors and transducers for strings, trees, graphs, 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 bidimentional patterns; cryptography; concurrency; cellular automata; bio-inspried computing; quantum computing.