This toolkit, produced for the G20 under Brazil's Presidency in 2024, is written to inform the Energy Transitions Working Group (ETWG) within the G20 Sherpa Track, as well as the wider international energy and climate change community. This publication provides an overview of the important role the IAEA plays in enhancing national and regional capacities for energy system analysis and planning in emerging markets and developing economies (EMDEs). This includes conducting capacity building activities with the transfer of energy planning tools and methodologies. These activities encompass distance and in-person training sessions, technical assistance, and addressing overarching energy strategies and transitions to low-carbon economies. The goal is to enhance knowledge and strengthen skills that many EMDEs currently lack, facilitating informed decision-making and policy development. The toolkit also showcases successful applications of its methodologies through case studies and collaborations with other international, regional, and national institutions, illustrating the positive impact of integrated energy planning on creating national policies and achieving sustainable development goals.