Global biogeochemical cycles can be defined as any of the natural circulation pathways through the atmosphere, hydrosphere, geosphere, and biosphere of the essential elements of living matter. These elements in various forms flow from the nonliving (abiotic) to the living (biotic) components of the biosphere and back to the nonliving again. Research on the biogeochemical cycles focuses on seven of the major elements that make up more than 95% of all living species: hydrogen, carbon, sulfur, oxygen, and the major nutrients nitrogen, phosphorus, and silicon. These elements combine in various ways. Here we will focus on carbon and sulfur due to their importance for feedback with the climate and for environmental concerns.