The purpose of instrumenting a fermentation process is two-fold, namely to understand and to control that process, from which it follows that the instrumentation is not an end in itself, merely a means to that end. Gaining a greater understanding of the process allows decisions to be made regarding the optimal operating conditions and follows from a study of the biological, chemical, and physical reactions which occur. Control of the process concerns the achievement of these optimal operating conditions. The common denominator of all activities aimed at understanding and controlling the fermentation process is the measured data and a few words concerning data characteristics are relevant to a discussion of instrumentation. For our present purpose, data may be considered to have 3 important characteristics, namely, its relevance, accuracy, and precision.