The topic of statistical ensembles is important in thermodynamics, statistical mechanics and quantum physics.

In quantum physics, there are two types of ensembles:

- Pure ensembles, which cannot be decomposed into different sub-ensembles. A ray in a Hilbert space is representing them in quantum mechanics.
- Mixed ensembles - decomposable into few different ensembles. A density matrix represents them in quantum mechanics. The decomposition into subensembles are many, and cannot be refined to a common one in general.

- density matrix
- Partition function (statistical mechanics)
- grand canonical ensemble