Rheological characterization of sludges represents one of the few examples of fundamentally derivable properties that has also been successfully related to actual sludge treatment processes. In understanding the relevance and utility of sludge rheology for practical applications, an historical perspective is of some initial value, and this paper summarizes previous developments in the rheological characterization of sludges and other non-Newtonian slurries, both from a theoretical and empirical perspective. In both cases, attention is brought to subtleties that must be considered if uniform and consistent rheological descriptions are to be obtained. Causes of the complex and variable rheological properties observed in sludges are then considered, with attention brought to particular aspects of upstream processes where some influence on sludge viscosity might be exerted. Finally, some practical aspects of rheometry are described. Recent research suggests that a variety of methods for sludge characterization and process optimization can be based on rheological measurements. Examples are presented that utilize both complex and relatively simplified technologies for this purpose, and some predictions are attempted of the probable directions for future developments. Internationally, approaches have differed and, while this diverse and competitive environment favors technological advances, the paper concludes by pointing out the need for consistency in rheological methods where fundamentally based parameters or regulatory concerns may be involved.