This book constitutes the refereed proceedings of the 21st International Conference on Computing and Combinatorics, COCOON 2015, held in Beijing, China, in August 2015. The 49 revised full papers and 11 shorter papers presented were carefully reviewed and selected from various submissions. The papers cover various topics including algorithms and data structures; algorithmic game theory; approximation algorithms and online algorithms; automata, languages, logic and computability; complexity theory; computational learning theory; cryptography, reliability and security; database theory,...