This chapter deals with different aspects of the crystal / amorphous alternative, notably the structural, dynamic and thermodynamic aspects. The glass transition at temperature Tg and its approach are presented with some details (Instability, aging, poly-amorphism, energy landscape etc.) Issues of solubility and stability, and ways to control them are particularly addressed. Finally, the means for preparing the amorphous pharmaceutical compounds, whether pure or in the form of dispersion, are described briefly.