Fundamental concepts of cake filtration - cake compressibility, liquid drag-pressure drop relationships, average porosity, and external mass balance - are presented. Two distinct approaches to the formulation of cake filtration rate, the classical and the modern theories, are critically reviewed. The definition of average specific cake resistance is re-examined. The equivalent specific cake resistance for a compressible cake is shown to be the harmonic mean of local specific cake resistances. Weaknesses of the classical theory in data analysis are pointed out. Recognition of the cake-filter medium interface by the modern theory as the rate-controlling part of the filtration system is stressed.