The safety of nuclear power plants (NPPs), one of the most important conditions for their acceptance, is achieved by numerous methods and techniques during their design, manufacture and maintenance. Risk informed inspection is one strategy for improving the effectiveness of in-service inspection (ISI) and is based on a combination of independent assessments of the probability of failure of components and the consequences of these failures. This publication describes the general process of developing and implementing risk informed in-service inspections (RI-ISI) methodologies, the technological issues which lie behind the methodologies, application and status, and the current development activities. The information provided is primarily intended for managers, ISI supervisors, lead engineers of NPPs and technical support organizations, but will also be useful for regulatory staff reviewing RI-ISI programmes.