This book constitutes the proceedings of the 24th International Conference on Developments in Language Theory, DLT 2020, which was due to be held in Tampa, Florida, USA, in May 2020. The conference was cancelled due to the COVID-19 pandemic. The 24 full papers presented were carefully reviewed and selected from 38 submissions. The papers present current developments in language theory, formal languages, automata theory and related areas, such as algorithmic, combinatorial, and algebraic properties of words and languages, cellular automata, algorithms on words, etc.