This book constitutes the refereed proceedings of the 30th International Colloquium on Automata, Languages and Programming, ICALP 2003, held in Eindhoven, The Netherlands in June/July 2003. The 84 revised full papers presented together with six invited papers were carefully reviewed and selected from 212 submissions. The papers are organized in topical sections on algorithms, process algebra, approximation algorithms, languages and programming, complexity, data structures, graph algorithms, automata, optimization and games, graphs and bisimulation, online problems, verification, the Internet, temporal logic and model checking, graph problems, logic and lambda-calculus, data structures and algorithms, types and categories, probabilistic systems, sampling and randomness, scheduling, and geometric problems.