Most of the conventional waste-water treatment processes yield large amounts of solid wastes called residual sludges. The rheological characterization of the raw primary, raw secondary and digested sludges is quite important because of pipe and pump systems design, sludge rheology implications in the mixing, mass and heat transfer processes on the digestion or conditioning operations, and finally, because of the possibility of employing apparent viscosity as a process control parameter. In the rheological characterization of the samples arising from different sources (i.e. raw primary and raw secondary sludges, and anaerobically or aerobically digested sludges), different values of apparent viscosity and flow indexes were found. Also, changes were detected during the anaerobic digestion process.