The International Standard Industrial Classification of All Economic Activities (ISIC) Revision 4 has been approved by the United Nations Statistical Commission as an international statistical standard. For the successful implementation of the revised classification of economic activities, a substantial amount of work has to be carried out to adapt business registers, business surveys, certain household surveys and general statistical programs including the national accounts. The national statistical offices will therefore face several major challenges in carrying out and coordinating this work. This manual provides national statistical offices (and other entities that are tasked with implementing the new classification) with general advice and specific recommendations, methodologies, common practices, check lists and similar tools that can be used during the complex project of implementing ISIC Rev. 4.