This book presents the proceedings of the Twentieth International Conference on Dependability of Computer Systems, showcasing recent advancements in this broad area. Contemporary computer systems and networks are the most complex structures ever engineered by man yet their reliable operation is paramount in today’s interconnected world. These systems form the backbone of almost every sector, from healthcare and finance to communication and transportation. Dependable systems ensure the seamless functioning of critical services, such as medical diagnostics, financial transactions, and emergency responses. This volume offers a selection of papers addressing challenges encountered in dependability studies of such systems. It can serve as an engaging and thought-provoking resource for scientists, researchers, engineers, and students who must tackle diverse dependability considerations in the design, analysis, or maintenance of contemporary computer systems. The 20th DepCoS-RELCOMEX conference marked yet another installment in a series of events held annually since 2006. Initially conceived as a platform for scholarly dialogue on reliability in computer engineering, the conference's focus has continually evolved to encompass emerging challenges arising from advancements in information technologies and computer engineering. Today, dependable computer operations involve delivering accurate and timely results while processing both quantitative and qualitative data, utilizing precise or fuzzy models and algorithms. As Artificial Intelligence and Large Language Models become increasingly prominent, ensuring dependability in modern IT and computer engineering necessitates employing cognitive systems and deep learning methodologies. The diverse topics explored in the conference papers underscore how crucial dependability has become across all applications of contemporary computer systems and networks. They also highlight the multifaceted, interdisciplinary nature of subjects that must be addressed in this area.