Reliability analyses for complex engineering systems inevitably involve uncertainties. Uncertainty management is essential for practical usability and credibility of reliability based decision making. An overview of the basic concepts of uncertainty propagation, together with simple and practical illustrations is presented. The issues of uncertainty management addressed in this book include: identifying the sources of uncertainty in reliability analyses, characterizing them at the element level, propagating them to the system level, recommending ways to handle the important contributors, and taking decisions in the light of uncertainties. All these issues are discussed with several case studies from nuclear power plants including electrical, electronic, and mechanical systems. This book is designed to assist practicing engineers, managers, and researchers involved in reliability and safety activities of complex engineering systems.