Stable isotopes of atmospheric carbon dioxide (CO₂) contain a wealth of information regarding biosphere-atmosphere interactions. The carbon isotope ratio of CO₂ (δ¹³C) reflects the terrestrial carbon cycle including processes of photosynthesis, respiration, and decomposition. The oxygen isotope ratio (δ¹⁸O) reflects terrestrial carbon and water coupling due to CO₂-H₂O oxygen exchange. Isotopic CO₂ measurements, in combination with ecosystem-isotopic exchange models, allow for the quantification of patterns and mechanisms regulating terrestrial carbon and water cycles, as well as for hypothesis development, data interpretation, and forecasting. Isotopic measurements and models have evolved significantly over the past two decades, resulting in organizations that promote model-measurement networks, e.g., the U.S. National Science Foundation's Biosphere-Atmosphere Stable Isotope Network, the European Stable Isotopes in Biosphere-Atmosphere Exchange Network, and the U.S. National Environmental Observatory Network.