This paper briefly reviewed the progress made recently to the estimation of hydrodynamic permeability of sludge flocs, which is essential for determining the extent of advection flow through the floc interior. The large pores mainly contribute the hydrodynamic permeability of sludge floc through which the fluid could readily flow. Permeability models that assume a homogeneous floc interior normally yield marked uncertainties in model outputs. Wu et al. (1998) proposed a hydrodynamic approach for estimating the hydrodynamic permeability that considers the response of the sludge floc when moves toward a flat plate. The floc permeability for waste activated sludge was noted higher than the cupric hydroxide sludge, while the polyelectrolyte flocculation and fluid shear could reduce the permeability. The bubble-tracking technique describes the fluid flow field around and through the floc interior. The existence of advection flow is noted unexceptional. The misuse of permeability model has been demonstrated with a case study leading to the floc porosity based on free-settling test.