A critical review of on-line mass spectrometry is presented, illustrating instrumentation, data treatment, and applications drawn from both control and research environments. Both direct and membrane inlets are included. The discussion of instrumentation focuses on commercially available options, although some research options that offer promise for future process applications are also mentioned briefly. Special considerations attendant upon the high-precision measurements needed for simultaneous analysis and continuous process control are emphasized. Applications illustrate a range of problems benefitting from rapid, simultaneous analysis of volatiles.