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This chapter addresses the biogeochemical cycles of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O). The three greenhouse gases have increased in the atmosphere since pre-industrial times, and this increase is the main driving cause of climate change (Chapter 10). CO2, CH4 and N2O altogether amount to 80% of the total radiative forcing from well-mixed greenhouse gases (Chapter 8). The increase of CO2, CH4 and N2O is caused by anthropogenic emissions from the use of fossil fuel as a source of energy and from land use and land use change, in particular agriculture. The observed change in the atmospheric concentration of CO2, CH4 and N2O results from the dynamic balance between anthropogenic emissions, and the perturbation of natural processes that leads to a partial removal of these gases from the atmosphere. Natural processes are linked to physical conditions, chemical reactions, and biological transformations and they respond themselves to perturbed atmospheric composition and climate change. Therefore, the physical climate system and the biogeochemical cycles of CO2, CH4 and N2O are coupled. This chapter addresses the present human-caused perturbation of the biogeochemical cycles of CO2, CH4 and N2O, their variations in the past coupled to climate variations, and their projected evolution during this century under future scenarios.