Sodium azide undergoes a transition from a high-temperature rhombohedral phase to a low-temperature monoclinic phase at 292.2 K. Results of x-ray diffraction studies of the low-temperature phase are reported which indicate that the transition is of the elastic type with the order parameter corresponding to a linear combination of the strains e13 and (e22−e11) along with a rotation of the azide ions in the xz plane. The temperature dependence of the order parameter in the vicinity of the transition is consistent with Landau theory for a weakly first-order change, and this model is also shown to explain a wide range of experimental data previously obtained on the transition. A microscopic model is developed on the basis of the quasi-harmonic approximation and this is shown to be in quantitative agreement with the experimental data on the transition.