The regulation of energy supply security represents a complex and evolving area of law, particularly in the context of the increasing integration of renewable energy sources. Contemporary energy markets are deeply interconnected and require a balanced and adaptive regulatory framework that addresses both conventional and renewable energy generation. While the long-term objective is the gradual phasing out of fossil fuels, existing energy systems continue to depend on them to ensure grid stability, particularly given the intermittent nature of renewable sources such as wind and solar, which often necessitate conventional backup to maintain supply reliability. Despite widespread consensus on the centrality of renewables in future energy systems, the transition toward a low-carbon energy framework remains impeded by significant regulatory shortcomings and coordination challenges. Realising a secure and sustainable energy future therefore demands more coherent, consistent and enduring regulatory interventions across all levels of governance. This book critically examines the roles of energy agencies and regulatory mechanisms operating at the global, regional and national levels, using Turkey as a case study, and assesses their current and prospective contributions to enhancing energy supply security and advancing the renewable energy transition.