The integrity of freeze-dried biological products after reconstitution, as well as their stability during storage, is often more sensitive to small variations in residual moisture level than traditional low molecular weight drugs. Susceptibility to over-drying has also been documented for biological products. This places additional demands on the quality of in-process monitoring to achieve the desired level of residual water. This discussion is a critical overview of methodology for in-process moisture determination, including product temperature measurement, comparative pressure measurement, pressure rise, and residual gas analysis. Advantages and limitations of these methods are discussed.