An important element of the milestones approach to introducing nuclear power is a mechanism to coordinate efforts among the many organizations and individuals who have roles to play in the process. This mechanism is referred to as a nuclear energy programme implementing organization (NEPIO). This publication describes a set of responsibilities, functions and activities that States can use as guidance for establishing a NEPIO and ensuring its effectiveness. This revision incorporates lessons learned from integrated nuclear infrastructure review missions and IAEA technical assistance activities. It attempts to clarify that there are many ways to structure a NEPIO and that each could result in the successful execution of all functions and activities. Several case studies are included. Consistent with the revision of IAEA Nuclear Energy Series No. NG-G-3.1 (Rev. 1) Milestones in the Development of a National Infrastructure for Nuclear Power published in 2015, this publication recognizes that the NEPIO plays an important and evolving role in each of the three phases of nuclear power infrastructure development.