This book constitutes the refereed proceedings of the 26th International Colloquium on Automata, Languages and Programming, ICALP'99, held in Prague, Czech Republic, in July 1999. The 56 revised full papers presented were carefully reviewed and selected from a total of 126 submissions; also included are 11 inivited contributions. Among the topics addressed are approximation algorithms, algebra and circuits, concurrency, semantics and rewriting, process algebras, graphs, distributed computing, logic of programs, sorting and searching, automata, nonstandard computing, regular languages, combinatorial optimization, automata and logics, string algorithms, and applied logics.