This book constitutes the refereed conference proceedings of the 26th International Colloquium on Structural Information and Communication Complexity, SIROCCO 2019, held in L'Aquila, Italy, in July 2019. The 19 full papers and 9 short papers presented in this book were carefully reviewed and selected from 39 submissions. They are devoted to the study of the interplay between structural knowledge, communication, and computing in decentralized systems of multiple communicating entities.