Global models of the Earth’s climate system are the primary tool scientists use to understand the Earth’s climate system. They yield critical insights into the components of the earth’s climate system, including the atmosphere, land, oceans, and biosphere, the processes at work within and between them, and how natural factors and human activities affect climate at the regional to global scale. This chapter summarizes the evolution of climate modeling and describes current global climate models and how they are being used to study the changing climate.