 Before delving into a discussion of these theories, let us briefly review the basic tenets of SM by dint of a common example. Consider a gas that is confined to the left half of a box. Now remove the barrier separating the two halves of the box. As a result, the gas quickly disperses, and it continues to do so until it uniformly fills the entire box. The gas has approached equilibrium. This raises two questions. First, how is equilibrium characterised? That is, what does it take for a system to be in equilibrium? Second, how do we characterise the approach to equilibrium? That is, what are the salient features of the approach to equilibrium and what features of a system make it behave in this way? These questions are addressed in two subdisciplines of SM: equilibrium SM and non-equilibrium SM.