The present chapter is about models. All the models are inspired by some physical system: sandpiles, earthquakes, magnetic vortex motion, forest fires, interface growth, or biological evolution. The models are defined in terms of a dynamical variable – for example, the local slope of the sand heap or the stress in the earthquake fault. The dynamical variable or field is updated in every time step according to some algorithm. The choice of the updating algorithm is, to some degree, arbitrary. The criteria for choosing the relevant definitions are, for the most part, simplicity and intuition. Statistical mechanicians have this overall belief that complexity arises from simplicity: that the intricate and complex ...