Water activity (Aw) is a measure of the activity of free or unbound water that is available to participate in undesirable chemical, physical and biological reactions. Measurement of water activity at multiple time points during the product life cycle will be related to changes in critical quality attributes such as degradation of the active ingredient, changes in the rate of dissolution or disintegration and changes in physical properties such as hardness or friability. Controlling water content in oral solid dose products (OSD) and dry pharmaceutical products in general, is essential to maintain efficacy and safety.