Solid-state fermentation plants, like many other processing facilities, are subjected to several kinds of disturbances, such as variations in raw materials, environmental changes, equipment failure, and human errors. If these disturbances are not detected and compensated properly, they will affect the operation and performance of the SSF plant, resulting in accidents, products out of specification, or loss of production. Hence, in order to operate the process reproducibly and safely, some relevant process variables should be measured (measured variables) and some others should be modified (manipulated variables), continuously or at least periodically. In this chapter we give some guidance on how to select relevant measured variables and about which instruments can be used to monitor SSF bioreactors. We will also discuss what to do when no instrument is available to measure a given variable and how to deal with noisy measurements. The selection of manipulation variables for control schemes is left for Chap. 28. We need to define our specific objectives in putting instrumentation on the bioreactor before selecting the variables to be measured and the instruments to make these measurements. In SSF bioreactors we should ensure that the microorganism grows well and produces the expected quantities of the desired metabolite. To achieve these in the specific case of packed-bed SSF bioreactors operated aseptically in batch mode, for example, we have to stop the process at the right time, detect undesirable heterogeneity in the solid bed, and avoid contamination by other microorganisms. We will emphasize the particular challenges that must be faced when making measurements inside a heterogeneous bed of solid particles.FONDECYT482FONDECYT