This article reviews slow dynamics of relaxation processes of glassy crystals in which the reorientational motion of the constituent molecules is frozen out, keeping the translational periodicity with respect to the centers of molecular mass. The glassy crystals around their glass transitions exhibit relaxational behavior which resembles closely those of glass‐forming liquids. Sub‐glass transitions occasionally appear below and above the primary Tg. In this way, the glassy crystals show universal features characteristic of the glassy liquids in all respects. Glassy crystals are thus promizing model substance for deeper understanding of the glasses in the sense that the translational invariance of the molecular arrangements renders their study more tractable.