This book constitutes the refereed proceedings of the 18th International Colloquium on Structural Information and Communication Complexity, SIROCCO 2011, held in Gdansk, Poland, in June 2011. The 24 revised full papers presented together with 1 survey lecture and 2 invited talks were carefully reviewed and selected from 57 submissions. The papers are organized in topical section on fault tolerance, routing, mobile agents, mobile robots, probabilistic methods, distributed algorithms on graphs, and ad-hoc networks.