A mature and well‑guided multi‑stage approach to the validation of nuclear power plant control room designs has the potential to reduce the risks involved in the design process. Such an approach can also increase the effectiveness of, and efficiencies in, the validation process, as well as overall confidence in the results. This relatively new concept of multi‑stage validation has yet to be defined in the technical literature, and thus this new NEA report describes the approach and the rationale for validating systems through a series of successive, co‑ordinated validation activities. The scope of application of multi‑scale validation addressed in this report includes aspects related to both the human factors engineering of new nuclear power plant main control room designs and modifications to existing control room designs. The objective is to provide a common reference for future dialogue, research and development concerning the multi‑stage validation approach, and ultimately to support the safe operation of nuclear power plants worldwide.