Book excerpt: This book constitutes the refereed proceedings of the 23rd International Conference on Runtime Verification, RV 2023, held in Thessaloniki, Greece, during October 3–6, 2023. The 13 full papers and 7 short papers presented in this book together with 4 tutorial papers and 2 invited papers were carefully reviewed and selected from 39 submissions. The RV conference is concerned with all aspects of novel lightweight formal methods to monitor, analyze, and guide the runtime behavior of software and hardware systems. Runtime verification techniques are crucial for system correctness, reliability, and robustness; they provide an additional level of rigor and effectiveness compared to conventional testing and are generally more practical than exhaustive formal verification.