In this paper an attempt is made to review the progress in membrane characterization and transport theory in historical perspective. Its central theme is the membrane "pore" around which progress has been revolving, irrespective of whether the researcher is "for" or "against" the existence of pores at the top dense layer of the RO membrane. The article starts from nineteen-fifties when the development of cellulose acetate membrane was launched on the basis of the Preferential Sorption-Capillary Flow (PS-CF) mechanism. The Sorption-Diffusion (S-D) model, which was presented at almost the same time, regards the top surface layer dense and homogeneous. No heterogeneity is allowed and the presence of pores makes the RO membrane imperfect, causing the leakage of salt. Thus, the PS-CF model came to direct confrontation with the S-D model. It is shown in this brief historic review how the advanced characterization instrument has revealed the heterogeneous structure of the top surface of the RO membrane and measured its "pore size" and "pore size distribution". The advanced transport theory based on Molecular Dynamics (MD) simulation also resulted in the presence of the multi-modal pore size distribution.