The technique to fabricate nanofibrous mat by electro-spinning has been known for a long time. But the attempts to use the electospun nanofiber mats, also known as electro-spun nanofiber membranes (ENMs), for filtration purposes began only recently. Among many membrane filtration processes, air cleaning by the removal of dust particles has already been commercialized and the product has been in the market for some time. On the other hand, the application of ENMs for liquid separation has a much shorter history and its commercialization has not yet been achieved. Since a large number of researches are reported in the open literature each year, its commercialization looks only a matter of time. For example, many papers are now available on the pressure driven membrane separation processes such as RO, NF, UF, MF by ENMs, and as many papers have been published on the other membrane separation processes including pervaporation, membrane distillation, forward osmosis and membrane adsorption. It is needless to say that EMFs have gained popularity within a short period due to the facile fabrication, interconnectivity and large area/volume ratio. Despite these advantages, ENMs’ pore sizes are intrinsically very large (fractions of micrometer to few micrometer), which makes modification of surface chemistry and especially reduction of the ENM pore size indispensable for wider applications of ENMs for membrane separation processes.