This book constitutes the refereed proceedings of the 30th International Colloquium on Structural Information and Communication Complexity, SIROCCO 2023, held in Alcalá 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 computing and communication. This is the 30th edition of SIROCCO, and 3 of the 26 papers in this book are devoted to celebrating this fact, plus an additional paper about a recent trend to study special models of computation.