This book constitutes the refereed proceedings of the 31st International Colloquium on Automata, Languages and Programming, ICALP 2004, held in Turku, Finland, in July 2004. The 97 revised full papers presented together with abstracts of 6 invited talks were carefully reviewed and selected from 379 submissions. The papers address all current issues in theoretical computer science including algorithms, automata, complexity, cryptography, database logics, program semantics, and programming theory.