This chapter examines the central role of fossil fuels in climate change and the urgent need for a fundamental shift in energy systems. It begins by distinguishing between primary and secondary energy, highlighting that fossil fuels still account for over 80% of global primary energy consumption. This chapter stresses that CO₂ emissions scale almost linearly with fossil energy use. It then discusses the promise and limitations of renewable energy, focusing on solar and wind, and identifies the need for energy carriers to address intermittency and transport issues. While batteries offer some solutions, they face significant limitations in cost, scale, and resource availability. This chapter introduces various chemical energy carriers, such as hydrogen, ammonia, methane, and especially methanol, and evaluates their suitability using quantitative criteria like energy density, infrastructure compatibility, and environmental impact. This chapter emphasizes that the selection of future energy carriers must be grounded in data and systems-level thinking.