The purpose of this paper is to dispel some of the more common myths that I have encountered over the years while talking to operators and working with them in countless treatment plants throughout the country. I use four notable myths as a jumping off point for the discussion that follows. In that discussion, I dispel other myths as well. The four notable myths are: 1) Increasing the RAS flow rate decreases the hydraulic detention time in the aeration basin, 2) The MLSS concentration is a function of the RAS flow rate, 3) You can control any two (or more) of the following parameters at the same time: sludge age (or SRT or MCRT), F:M ratio, and MLSS/MLVSS concentration (or total mass inventory), and 4) The key process control focus of activated sludge operators is the MCRT.