Reaching the environmental, economic and social sustainability objectives of all societies requires that several major energy challenges be successfully overcome and necessitates rapid progress along multiple dimensions. This requires a paradigm shift, including the fundamental transformation of the energy system. Energy systems, from supply to services, will need to be re-designed to provide universal access to energy for the poor while at the same time limiting air pollution and health damages from energy use, improving energy security throughout the world, and limiting climate change. Systems analyses at IIASA utilizing integrated, systems-level modeling tools can help to shed light into complex interactions that underpin most of these critical issues. This talk focuses on one of the major research projects in that vein: the recently completed analysis of future energy pathways at IIASA. The results from the study indicate that the transformation towards ambitious sustainability objectives is technically possible, and that alternative combinations of resources, technologies, and policies are conceivable to attain the objectives. The research, carried out under the umbrella of the Global Energy Assessment (GEA), identifies technical measures, policies, and related costs and benefits of the energy transformation.