Global geochemical cycles are the processes that control the chemical composition of the environment, particularly the ocean and the atmosphere. The emphasis in this book is on the theory of global geochemical calculation, the methods used to calculate the composition of the environment, and exploration of changes that might occur in time. The goal is to enable Earth scientists to set up their own geochemical models; to explain how to represent a geochemical system in mathematical terms; and how to solve the systems of equations that typically arise in geochemical modelling.