This book presents an overview of state-of-the-art approaches to determine thermal safety margins in nuclear reactors. It presents both the deterministic aspects of thermal safety margins of nuclear reactors and a comprehensive treatment of aleatory and epistemic uncertainties to facilitate the understanding of these two difficult topics at various academic levels, from undergraduates to researchers in nuclear engineering. It first sets out the theoretical background before exploring how to determine thermal safety margins in nuclear reactors, through examples, problems and advanced state-of-the-art approaches. This will help undergraduate students better understand the most fundamental aspects of nuclear reactor safety. For researchers and practitioners, this book provides a comprehensive overview of the most recent achievements in the field, offering an excellent starting point to develop new methods for the assessment of the thermal safety margins. This book is written to bridge the gap between deterministic methods and appropriate treatment of uncertainties to assess safety margins in nuclear reactors, presenting these approaches as complementary to each other. Even though these two approaches are frequently used in parallel in real-world applications, there has been a lack of a consistent teaching approach in this area. This book is suitable for readers with a background in calculus, thermodynamics, fluid mechanics, and heat transfer. It is assumed that readers have previous exposure to such concepts as laws of thermodynamics, enthalpy, entropy, and conservation equations used in fluid mechanics and heat transfer. Key Features: • Covers the theory, principles, and assessment methods of thermal safety margins in nuclear reactors whilst presenting state-of-the-art technology in the field • Combines the deterministic thermal safety considerations with a comprehensive treatment of uncertainties, offering a framework that is applicable to all current and future commercial nuclear reactor types • Provides numerous examples and problems to be solved.