Monitoring is an important exercise in the operation of submerged bioprocesses and is a subject that has attracted intense research activity in recent years. In order to achieve optimal production or conversion, the factors that influence the performance of a bioprocess should be measured, preferably on-line, so that the process can be monitored in real-time and appropriate control or remedial action implemented. Such measurements can be achieved either in situ or ex situ. Ideally, in situ approaches are desirable. Where such an option is difficult to implement, ex situ approaches with appropriate sample handling techniques can be employed. In cases where direct analytical information is not available, inferential approaches can be adopted. In addition, analytical information not yet accessible on-line can be obtained off-line. Computational techniques have enabled the compilation of measurement data before utilizing the information for process diagnosis. Current developments in the different measurement technologies are presented with a critical assessment of their potential application in industrial production or process development.