The factors affecting the growth of most common filamentous microorganisms are discussed. In a design procedure the following factors should be considered: wastewater composition (readily and slowly biodegradable substrates, inoculation), biomass retention time, actual substrate concentration in reactor, operational parameters in reactor (DO and nutrients concentration, pH, temperature) and cultivation conditions. The configurations of activated sludge process supporting the growth of floc-formers are described. As general design methods and criteria for the design of bulking control systems are not available due to the complexity of selection mechanisms, the technological parameters from successful bulking control case histories are summarized. The possibilities of mathematical activated sludge process models for design purposes are evaluated.