A statistical description involves the notion of distribution functions of the various quantities that allow establishing average values that are linked to physical observables. In this chapter, the case of classical systems is addressed. Distributions in phase space are considered, together with the equations they satisfy, such as the Liouville’s theorem. The concept of ensembles and their associated probability distributions is introduced, and their equivalence in the thermodynamic limit is established. The third law of thermodynamics emerges from this statistical description. The classical information theory and its link to statistical properties is introduced using Jaynes formalism.