The Gateway Review process aims to deliver a peer review of projects at critical stages in their lifecycle, to provide assurance that they can progress successfully to the next stage. The Gateway Review Process covers six Gates , numbered from 0 to 5. These gateways cover aspects from strategic assessment of the program to examining the full business case of the project, as well as monitoring the operations of a project. Principles of the Green Book are incorporated in Gates 1 and 2. For all major projects, HMT approvals are required at Gates 1, 2 and 3.