The aim of this monograph is to familiarize the reader with a complete mix activated sludge pilot reactor. It is by no means comprehensive and should thus work only as a starting point. However, by reading this “guide” the reader would be able to build, operate, control, and maintain an activated sludge pilot reactor, which could be used for different studies. Some parameters for the operation of the pilot have been set according to its dimensions. As a consequence the pilot should be built as described in this paper. These particular parameters are thus not described in detail because they require a considerable amount of study. The pilot has been used for a study, which is not covered in this work due to a confidentiality agreement. No results will be showed, only few examples whenever possible. My supervisor, Dr. Raghida Lepistö and Tampere University of Technology, Tampere, Finland, disclaim any responsibility for the content of this work.