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This paper reports a new approach to understanding membrane fouling in membrane bioreactors (MBR). The foulants were categorized into those inside the membrane (internal fouling) and those on the membrane forming the so-called external fouling layer (external fouling). The volumes of the internal and external foulants were determined with the aid of a confocal laser scanning microscope (CLSM) after staining the foulants. Thus, the porosities of both the membrane and the external fouling layer were calculated, which explained the contributions of internal and external fouling to transmembrane pressure (TMP) from the standpoint of fundamental filtration theory. The categorical method indicates that external fouling may dominate the long-term operation of MBR with the formation of the external fouling layer, whereas internal fouling, in the form of pore constriction, may cause both initial and final TMP jumps under the operating conditions in the current study. In addition, a mathematical model was developed to dynamically link TMP with the basic filtration parameters (e.g.,porosities of the membrane and external fouling layer, thickness of the external fouling layer, pore size inside the membrane, and size of the particles forming the external fouling layer). The model quantitatively describes the contributions of both internal and external fouling and further supports the experimental results.