Technology readiness levels (TRLs) provide a standardized and objective method for evaluating the maturity of different technologies and their different components, making it easier for all stakeholders to understand and use. By using TRLs in specific technology programme planning, everyone from government and research organizations to private sector developers, end-users, and the supply chain can benefit from a consistent and transparent evaluation process. This publication addresses the growing need for the use of TRLs in fusion programmes, and it is expected that their use will continue to increase in the future. This publication is intended for technologists, researchers, university professors and students, regulatory experts, nuclear engineers, and nuclear fusion plant planners.