High rate anaerobic treatment reactors are able to uncouple solids and liquid retention time, resulting in high biomass concentrations. Principal advantages of anaerobic treatment include: energy efficiency, low biomass yield, low nutrient requirement and high volumetric organic loadings. In order to facilitate small reactors operation with stable and good performance automatic process control systems in combination with on-line instrumentation are proposed. The paper reviews the development and availability of the principal instrumentation for anaerobic treatment processes. First, the most important measuring principles are discussed, followed by a review of the most important process variables with emphasis on the development of their instrumental measurement techniques and application in research. Finally, a summary of actual application of instrumentation in full-scale anaerobic treatment plants is presented.