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This section describes the application of headspace gas analysis, specifically frequency modulation spectroscopy (FMS), to non-destructive measurement of moisture content in freeze dried product. Water is present in a lyophilized cake in a variety of forms, including free, adsorbed, chemically bound, and water of crystallization. Not all of these water forms are linked to product stability through degradation of excipients and active ingredients. Generally, free or active water is available for chemical reaction and has the greatest impact on product stability. Traditional moisture analysis methods for pharmaceutical products such as Karl Fischer (KF) titration and thermo-gravimetric analysis (TGA) do not distinguish between active water and bound water. For example, KF measures the total water in the freeze dried cake if the sample is wholly soluble in the KF medium. From a practical point of view, the KF and TGA methods are time consuming, involve chemical reagents, and destroy the sample. Aseptic processing would benefit from non-destructive measurement technologies that provide information on each individual vial not just a small statistical sampling. Testing entire batches can provide insight into the efficiency of the drying process and the dynamics of freeze drying in different locations within the freeze dryer. In addition, quality control groups would benefit from being able to non-destructively sample a statistically significant number of finished product vials as part of a release test as well as monitoring individual vials repeatedly over the product shelf life. Finally, 100% moisture inspection of finished vials in manufacturing would guarantee the quality of finished product with respect to stability against moisture.