This book constitutes the refereed proceedings of the 9th International Conference on Developments in Language Theory, DLT 2005, held in Palermo, Italy in July 2005. The 29 revised full papers presented together with 5 invited papers were carefully reviewed and selected from 73 submissions. All important issues in language theory are addressed including grammars, acceptors, and transducers for strings frees, graphs, and arrays; efficient text algorithms; algebraic theories for automata and languages; variable-length codes; symbolic dynamics; decision problems; relations to complexity theory and logic; picture description and analysis; cryptography; concurrency; DNA computing; and quantum computing.