Mesospheric thermal inversions are investigated in a numerical simulation with the Whole Atmosphere Community Climate Model, an upward extension of the National Center for Atmospheric Research's Community Climate Model. The seasonal character, spatial extent, and magnitude of the inversion layers are realistic during winter. In the model, the occurrence of wintertime inversions is a direct consequence of the rapid decay with height of vertically propagating planetary waves, which induces large temperature perturbations in the upper mesosphere to maintain hydrostatic equilibrium. The magnitude of the inversions is highly correlated with planetary wave amplitude, so that large inversions develop during episodes of planetary wave amplification. Gravity waves do not play a major direct role in the formation of the inversions because the largest thermal tendencies associated with gravity wave breaking occur well above the range of altitudes where inversions are found. However, gravity waves play an essential indirect role because they set up a critical line in the upper mesosphere where Rossby waves break in the mesospheric surf zone.