INTRODUCTION It has now been amply demonstrated that the different lattice energies (and entropies) associated with different polymorphs or solvates give rise to measurable differences in the physical properties (density, color, hardness, refractive index, conductivity, melting point, enthalpy of fusion, vapor pressure, etc.). Even the explosive power of cyclotetramethylene-tetranitramine depends on which of its four polymorphs is being used (1). We have seen in previous chapters that the different lattice energies of polymorphs or solvates give rise to different solubilities and dissolution rates. If the solubilities of the various solid forms are suffi ciently different, they can be very important during the processing of drug substances into drug products (2), and may have implications for the adsorption of the active drug from its dosage form (3). These concerns have led to an increased regulatory interest in the solid-state physics of drug substances, and in their characterization (4-10).