This document summarizes the key biogeochemical cycles: the carbon, water, nitrogen, oxygen, phosphorus, and sulfur cycles. It provides brief descriptions of each cycle, explaining how the chemical element or compound moves through the biosphere, hydrosphere, and lithosphere. The carbon cycle involves the movement of carbon between organisms through photosynthesis, respiration, and combustion. The water and nitrogen cycles similarly describe the circulation of water and nitrogen between living things and the environment.