This book constitutes the refereed proceedings of the 7th International Conference on Developments in Language Theory, DLT 2003, held in Szeged, Hungary, in July 2003. The 27 revised full papers presented together with 7 invited papers were carefully reviewed and selected from 57 submissions. All current aspects in language theory are addressed, in particular grammars, acceptors, and transducers for strings, trees, graphs, arrays, etc; algebraic theories for automata and languages; combinatorial properties of words and languages; formal power series; decision problems; efficient algorithms for automata and languages; and relations to complexity theory and logic, picture description and analysis, DNA computing, quantum computing, cryptography, and concurrency