Modern geochemistry aims to provide an accurate description of geological processes, and a set of models and quantitative rules that help predict the evolution of geological systems. This work is an introduction to the mathematical methods of geochemical modeling, largely based on examples presented with full solutions. It shows how geochemical problems, dealing with mass balance, equilibrium, fractionation, dynamics, and transport in the igneous, sedimentary and oceanic environments, can be reformulated in terms of equations. Its practical approach then leads to simple but efficient methods of solution. This book should help the motivated reader to overcome the formal difficulties of geochemical modeling, and bring state-of-the-art methods within reach of advanced students in geochemistry and geophysics, as well as in physics and chemistry.