This introductory chapter situates clean hydrogen within the wider context of the global drive towards net-zero carbon emissions. It outlines the scale and structure of the decarbonisation challenge, identifying hard-to-abate sectors where electrification alone is insufficient and hydrogen may provide unique value. The chapter traces the evolution of hydrogen’s role in energy systems, highlighting the technological, economic, and policy factors that have shaped both past disappointments and current optimism. It introduces the book’s central questions—when, where, and how hydrogen truly matters—and argues that its contribution will depend less on universal deployment than on targeted, system-wide integration. In doing so, the chapter sets the stage for a critical yet pragmatic examination of why clean hydrogen will not become the ‘next oil’, but why it still holds strategic importance for decarbonisation.