A nuclear power plant design is the product of the activities of many organizations, and changes to that design will occur due to improvements or technical evolution continuously over the plant's lifetime. Changes can lead to a deterioration, or a loss of original design basis caused by inadequate configuration control processes. This necessitates a reconstitution of the design basis of systems, structures, and components to sustain the safe, reliable, and efficient operation and the plant performance. This publication presents essential elements such as the drivers, goals, methods, roles, responsibilities and interfaces for conducting an effective design basis reconstitution. It also describes current challenges, operating experience, good practices, and lessons learned associated with design basis reconstitution for long term operation. The publication has been written for all those involved in assessing, updating, and implementing design basis reconstitution practices.