Before describing individual geothermal fields in Japan in the following chapters, the first half of this chapter will provide fundamental knowledge of the tectonic background in the Japanese islands, such as tectonics and magma generation, from a nationwide viewpoint. Seismic zones, seismic tomography, and low-velocity zones visualize the Japanese islands’ subduction regime and magma generation. The individual roles of the Pacific and the Philippine Sea slabs are described in the slab-overlapped area in central Japan, where the Philippine Sea slab is an insulator from the upper hot mantle wedge to the lower cold Pacific slab. Deep low-frequency earthquakes enable the tracing of replenishment conduits of basaltic magma from the upper mantle to the subvolcanic magma chambers in some volcanoes in Japan. The latter half of this chapter examines Japan’s geothermal development, highlighting historical milestones, government policies like the feed-in tariff (FIT), and ongoing R&D efforts. It addresses technical challenges such as supercritical geothermal resources and the need for cooperation with local hot spring communities to achieve Japan’s carbon–neutral goals by 2050. The chapter also presents a practical approach to geothermal exploration, emphasizing the importance of accurate reservoir characterization and modeling. It introduces the “Three Key Elements for Geothermal Systems” (3KEYGS) and discusses their role in conceptual models. Additionally, it covers the stages of geothermal power development, focusing on the integration of reservoir simulation for economic viability.