General Circulation Models (GCMs) are mathematical models that attempt to simulate a planet's climate system. They require many thousands of calculations which are done using fast computers. In many ways, GCMs resemble the computer models that are often seen on television weather forecasts. GCMs lie at the upper end of the climate model hierarchy, which range in complexity from simple calculations of a single quantity such as average global planetary temperature, to models that can predict changes over time of quantities such as temperature, humidity, and winds anywhere in the atmosphere in three dimensions (height, latitude, longitude). The mathematical equations that comprise a GCM can be loosely broken down into three separate but linked categories: (1) the forces in the atmosphere (called the "dynamics") that describe the large-scale movement of air masses and transport of energy and momentum; (2) the physics of the climate system such as radiation transmission through the atmosphere, heating changes, and conservation of energy; and (3) other factors such as topography and soil properties. Equations are based on known physical laws such as the conservation of energy and mass, as well as relationships based on observations such as those between temperature and humidity changes in cloud formation.

For Mars, GCMs are important in understanding the small amount of observations that we have of the Martian atmosphere and in predicting atmospheric properties for which we have no data. They can also be used with certain parameters changed (such as the axial tilt of the planet) to simulate the climate of Mars in the past. This helps to assess changes in the global environment through geological time.