In solid state physics and quantum mechanics, a function quantifying how many distinct energy states exist at (or very near) a given energy level.

A high density of states indicates either very small energy gaps or a high degree of degeneracy. Statistical mechanics allows calculation of internal energy from the density of states for electrons and phonons. This in turn allows calculation of such thermodynamic functions as heat capacity, entropy, Gibbs free energy and Helmholtz free energy of the material.

The electron density of states is also somewhat useful in calculating the Fermi level and electrical conductivity of a material.