Sludge dewatering is a difficult process in water and wastewater treatment, as commonly achieved through filtration practice. The purpose of sludge dewatering is to remove as much of the liquid portion of the sludge so that its transport and disposal becomes possible and feasible. The extent of the performance of the filtration operation is highly influenced by the characteristics of the sludge; such as its solids concentration, bound water content, organic content, cellulose content, pH, electrical charge of particles and other sludge specific parameters. To assess the filterability and dewaterability of sludges, parameters like specific cake resistance, capillary suction time and Buchner funnel filtration time are used. The most commonly used parameter is the average specific cake resistance evaluated by the parabolic filtration equation which was developed by Ruth in 1933 employing a heuristic analogy with Ohm's Law. However, it is noted that the average specific cake resistance is not a correct parameter to characterize the filtration characteristics of many types of sludges. Because operational parameters such as, filtration area, pore size of the filter medium, applied vacuum pressure and mode of filtration operation are reported to affect the average specific cake resistance. Besides the conventional filtration theory, in literature three major theories are proposed to define filtration. However, all these three approaches are not practically applicable to characterize the filterability and dewaterability of sludges. This study is a brief review of the current status of the filtration theory.