This book constitutes the refereed proceedings of the 30th International Colloquium on Structural Information and Communication Complexity, SIROCCO 2023, held in Alcala de Henares, Spain, during June 6-9, 2023. The 26 full papers presented in this book were carefully reviewed and selected from 48 submissions. SIROCCO is devoted to the study of the interplay between structural knowledge, communication, and computing in decentralized systems of multiple communicating entities. Special emphasis is given to innovative approaches leading to better understanding of the relationship between...