A model is a representation of an idea, a process or a system that is used to describe natural phenomena that cannot be experienced directly. In the Earth Sciences one uses models because the knowledge of natural processes is often quite incomplete, but also because the knowledge can involve details of such complexity so that simplifying assumptions are necessary in order to make the problem tractable with physical or chemical theories. There exist a variety of models ranging from conceptual models (e.g. carbon cycle, mass extinctions), geometric models (e.g. geological maps and cross sections), to thermodynamic models (e.g. subduction, seismic wave propagation or surface processes). The entire range of models, from purely conceptual to physics-based models, is important for the Earth Sciences to explain, quantify and eventually predict natural processes.