This book constitutes the refereed proceedings of the 21st International Colloquium on Structural Information and Communication Complexity, SIROCCO 2014, held in Takayama, Japan, in July 2014. The 24 full papers presented together with 5 invited talks were carefully reviewed and selected from 51 submissions. The focus of the colloquium is on following subjects Shared Memory and Multiparty Communication, Network Optimization, CONGEST Algorithms and Lower Bounds, Wireless networks, Aggregation and Creation Games in Networks, Patrolling and Barrier Coverage, Exploration, Rendevous and Mobile Agents.