A theory of the specific heat capacity of solids put forward by Peter Debye in 1912, in which it was assumed that the specific heat is a consequence of the vibrations of the atoms of the lattice of the solid. In contrast to the Einstein theory of specific heat, which assumes that each atom has the same vibrational frequency, Debye postulated that there is a continuous range of frequencies that cuts off at a maximum frequency ν‎_D, which is characteristic of a particular solid. The theory leads to the conclusion that the specific heat capacity of solids is proportional to T³, where T is the thermodynamic temperature. This result is in very good agreement with experiment at low temperatures.

A key quantity in this theory is the Debye temperature, θ‎_D, defined by θ‎_D = hν‎_Dk, where h is the Planck constant and k is the Boltzmann constant. The Debye temperature is characteristic of a particular solid. For example, the Debye temperature of sodium is 150 K and the Debye temperature of copper is 315 K. Although there are more accurate theories of the specific heat of solids at low temperature than Debye’s, such theories are much more complicated.