This chapter frames the energy transition and why the ocean matters. It traces the historical dominance of fossil fuels, underscores the urgency through climate impacts and the limited near-term role of carbon capture, and highlights the recent growth of renewables. It then positions ocean-based generation as a necessary complement to land-based build-out, unlocking space and resource diversity, and sets the book’s scope. It introduces current and emerging ocean energy technologies and their integration into the wider ocean ecosystem, and examine contemporary challenges and pathways to scale. The chapter closes by outlining the structure of the book and the audiences it serves.