This chapter establishes the conceptual and empirical foundations for analyzing environmental technology innovation using patent data. It begins by underscoring the strategic importance of environmental technologies in achieving sustainable development, highlighting major policy frameworks including Japan’s Innovative Environmental Innovation Strategy, the European Green Deal, and post-COVID green recovery programs. A systematic review of the literature then distinguishes between two research streams: studies focusing on technology development, which predominantly use econometric models with patent counts as dependent variables, and those addressing technology diffusion, which leverage patent citations and international registration patterns. The chapter introduces and evaluates a range of measurement approaches—from R&D expenditure to patent counts, licensing fees, and trade data—demonstrating that patent data occupy a uniquely favorable position by combining accessibility, standardized classification, and citation-based diffusion traceability. The OECD’s environmental patent classification system and the IPC-based taxonomy developed by Haščič and Migotto (2015) are presented as practical identification tools. The chapter concludes by articulating the book’s objective: to examine shifts in R&D strategies across five major patent offices (CNIPA, USPTO, EPO, JPO, KIPO) in three critical domains—renewable energy, environmentally friendly vehicles, and pollution control—using LMDI-based patent decomposition analysis. This framework provides policymakers, corporate strategists, and researchers with a data-driven compass for evidence-based decision-making.