The key process engineering differences between reverse osmosis (RO) forward osmosis (FO) and pressure retarded osmosis (PRO) are emphasized. Attention is drawn to the importance of mass transfer limitations which are more severe in FO than in other membranes as module size increases. Comprehensive general equations are developed and applied to selected recent data. The supposedly linear performance with increasing draw solution concentration mentioned by some is incorrect. The maximum possible water flux through a FO membrane used in a desalination application is proportional to LN(pi(d)/pi(f)), where pi(d) and pi(f) are the osmotic pressures of the draw solution and the saline feed water, respectively. Scepticism is expressed as to whether FO will ever contend with RO for the production of potable water by sea water desalination because the strength of the draw solution for comparable fluxes to RO is given by pi(d)congruent to Delta P exp(J(v)/k(op)) where Delta P is the TMP in RO. The future of FO probably lies with the dewatering of very high salinity brines, niche applications and possibly PRO. (C) 2013 Elsevier B.V. All rights reserved.