Physical systems can be split into three categories: classical mechanics, which describes machines with a finite number of moving parts, quantum mechanics, which describes the structure of atoms, and statistical mechanics, which describes gases, liquids, solids; this includes condensed matter physics. The case of classical mechanics is discussed in the next section, on ergodicity in geometry. As to quantum mechanics, although there is a conception of quantum chaos, there is no clear definition of ergodocity; what this might be is hotly debated. This section reviews ergodicity in statistical mechanics.