High-rate anaerobic treatment has emerged as a viable alternative for the treatment of many industrial and municipal wastewaters. A number of different process options have been reduced to practice, although some configurations are clearly more well developed than others. One common thread that links these various processes (principally the Anaerobic Filter, Upflow Anaerobic Sludge Blanket and Expanded/Fluidized Bed Reactors), is the ability to effectively separate solids and hydraulic retention times. This permits design to be based upon the degradative capacity of the anaerobes, not growth rate and results in reduction of treatment times from days (typical for conventional digester systems) to hours. This article compares and contrasts the principles of start-up and operation of these different high-rate anaerobic systems based upon laboratory research and full-scale operating experience gained over the past two decades. The application of anaerobic processes for treatment of certain toxic and hazardous waste streams is just beginning. The limited work performed to date and anticipated future needs for process monitoring and control are also presented in this article.