On-line mass spectrometry (MS) has been developed and implemented as a tool to monitor and quantify a number of PAT & QbD applications. These include monitoring and control of fermentors, bioreactors and photobioreactors for the production of therapeutics, vaccines, industrial enzymes, biofuels and food additives. MS has also been effectively deployed for solvent drying control in pharmaceutical formulation and in direct human calorimetry where nutrition and metabolism studies require highly precise gas composition measurement. The use of online MS for QbD process development will be discussed with a number of real-world examples shown.