A total of 204 pairs of different crystal structures for the same organic molecule (polymorphs), determined at room conditions, were retrieved from the Cambridge Structural Database. Crystallographic, chemical, and pharmaceutical aspects of the phenomenon were considered. Correlations between differences in density, calculated packing energy, and lattice-vibrational entropy, and other crystal properties, are presented. Indices to quantify conformational polymorphism and differences in coordination sphere in the crystal are proposed. Differences in lattice-vibrational entropy between polymorphs are seldom, if ever, large enough to equal or to exceed differences in packing energy (enthalpy) at room temperature. Although few experimental estimates of energy differences between polymorphs are available, the overall results and some detailed comparisons with calculated lattice energies confirm the good performance of the parameters of the crystal potential. A tentative polymorph for aspirin is proposed by a structure generation procedure. The occurrence of polymorphism in organic crystals is very frequent, if the proper temperature range is explored, but at room conditions, the appearance of several polymorphic forms is not as pervasive as it is sometimes said to be.