Fundamental changes in our understanding of the complex behaviour of the atmosphere and climate processes have been obtained over the past three decades through the development and application of numerical climate and weather models. Numerical models simply use mathematical relationships to describe physical processes. There are many forms of climate and weather models ranging from simple point energy balance approaches to three-dimensional general circulation models (GCMs) which attempt to model all the complexities of the earth climate system. We discuss in more detail the GCM in its various forms which is used to simulate both climate and weather for day-to-day forecasting.