This monograph presents and discusses risk importance measures as quantified by the probabilistic risk assessment (PRA) models of nuclear power plants (NPPs) developed according to the current standards and practices. Usually, PRA tools calculate risk importance measures related to a single "basic event" representing particular failure mode. This is, then, reflected in many current PRA applications. The monograph focuses on the concept of "component-level" importance measures that take into account different failure modes of the component including common-cause failures (CCFs). In opening sections the role of risk assessment in safety analysis of an NPP is introduced and discussion given of "traditional", mainly deterministic, design principles which have been established to assign a level of importance to a particular system, structure or component. This is followed by an overview of main risk importance measures for risk increase and risk decrease from current PRAs. Basic relations which exist among the measures are shown. Some of the current practical applications of risk importance measures from the field of NPP design, operation and regulation are discussed. The core of the monograph provides a discussion on theoretical background and practical aspects of main risk importance measures at the level of "component" as modeled in a PRA, starting from the simplest case, single basic event, and going toward more complex cases with multiple basic events and involvements in CCF groups. The intent is to express the component-level importance measures via the importance measures and probabilities of the underlying single basic events, which are the inputs readily available from a PRA model and its results. Formulas are derived and discussed for some typical cases. The formulas and their results are demonstrated through some practical examples, done by means of a simplified PRA model developed in and run by RiskSpectrum® tool, which are presented in the appendices.