The nature of molecular or macromolecular assembly in terms of crystalline or amorphous packing in the solid state inf luences several critical attributes of the material. It controls physical properties of polymers such as the glass transition temperature, viscoelastic nature and electrical responses. In the case of small molecule based materials, it can impact upon characteristics such as the solubility af f ecting pharmaceutical applications, mechanical hardness, charge carrier mobility, and photophysical responses. The reversible transf ormation between the amorphous (A) and crystalline (C) f orms is key to the f unctioning of important classes of phase change materials (PCMs). A f undamental understanding of the molecular structural f eatures that f avor the A or C supramolecular assembly and a delicate balance between them enabling reversible transf ormations, is of paramount interest in the design of f unctional molecular materials with tunable crystallinity, or ef f icient phase change systems. However, such correlations have rarely been established in detail; examples of some reports, with their scope limited to isolated s y stems or specif ic experimental or analytical investigation, are the f ollowing. In the realm of inorganic solids, it has been shown that the P:Al ratio controls the f ormation of A and C aluminophosphates with the f ormer dominating at ratios less than 1.0. Interesting cases in the domain of molecular solids are the promotion of the A phase in 1,3,6-substituted f ulvene molecular glasses by the bulky substituents on the f ulvene ring, and the molecular conf ormation inf luencing the crystallinity and luminescence of persulf urated benzenes. The extent of crystallinity in poly-mers is controlled by several molecular f eatures and inter-macromolecular interactions.