Stable isotope compositions of biologically cycled elements encode information about the interaction between life and environment. On Earth, geochemical biomarkers have been used to probe the extent, nature, and activity of modern and ancient organisms. However, extracting biological information from stable isotopic compositions requires untangling the interconnected nature of the Earth’s biogeochemical system, and must be viewed through the lens of evolving metabolisms on an evolving planet. In this chapter, we provide an introduction to isotope geobiology and to the geobiological history of Earth. We discuss the isotope biogeochemistry of the biologically essential elements carbon, nitrogen and sulfur, and we summarize their distribution on the modern Earth as an interconnected network of isotopically fractionated reservoirs with contrasting residence times. We show how this framework can be used to explore the evolution of life and environments on the ancient Earth, which is our closest accessible analogue for an extraterrestrial planet.