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The study of nonisothermal flux of air through porous media dates back to 1873, whereas the existence of a nonisothermal liquid transport through membranes was first described in 1907. This phenomenon termed thermo-osmosis (TO) did not involve any liquid/vapor phase transition and was carried out through both dense and porous hydrophilic membranes. About 50 years later, when porous hydrophobic membranes were used and the nonisothermal vapor transport was studied through dry pores, the phenomenon was known as membrane distillation (MD). This nonisothermal membrane separation process is applied mostly in desalination and for the treatment of different types of wastewaters including brines for water production. It was known 50 years ago but has only recently made its way toward industrial applications. It still needs to be improved further in various key aspects. Compared with TO, much more …