The value of models to explain growth responses of filamentous microbes in activated sludge is well recognised. A much better understanding of the conditions which promote both bulking and foam formation is now available, as are the methods by which sludge instability can be controlled. What is missing however, is the order by which corrective measures should be implemented. This is one which can be clearly understood by both the operators of the activated sludge process and its designers. This paper offers a logical route by which decisions can be made, presented in order of the effort and the cost required to effect a cure. Process options merely involve valve and aerator adjustments, whereas operational modifications need minor alterations to the plant. The most expensive alternatives are the engineered solutions involving the design and construction of additional reactors, such as selectors. A systematic rather than symptomatic approach to bulking and foaming, using firm data and decision pathways, is a pre-requisite for any modelling exercise or process simulation.