Most structural studies of polymeric phase inversion membranes have focused on macrovoid formation, while the rich microstructure (size scale < 5 μm) has received little independent attention. We present a classification and qualitative description of possible microstructures on the basis of phase diagram, kinetic, and connectivity considerations. Special attention is given to the microstructural features resulting from a two-stage phase separation. Subsequently, our considerations are extended to the microstructure of the skin and that of the macrofinger boundaries. Finally, the features of actual phase inversion microstructures are described and interpreted.