The IAEA's Framework for Modelling Energy Systems (FRAMES) consists of an analytical model and a tool designed for energy system modelling. It supports Member States in evaluating how nuclear energy can contribute to reliable, low carbon and cost optimized energy systems. The new TECDOC provides a detailed description of the framework and the associated FRAMES Model and its use in modelling and comparing energy system options, integrating nuclear with renewables, storage and other technologies, and informing long term energy planning.