The non-isothermal membrane distillation (MD) separation process is known for about 50 years and very few studies are reported on its economics, energy analysis and costs evaluations. Dispersed and confusing water production costs (WPC) and specific energy consumption (EC) analysis were reported. Most of them are simulated and others are based on various costs assumptions. At present, the common asked questions about the published papers in MD including EC and WPC are: how these reported calculations on WPC and EC were made?, what is the current WPC of MD?, and how WPC of MD can be improved?. An overview of most studies carried out on these issues is presented and some useful equations and information in this context are reported. Comparison to other separation processes used in desalination is made. At present, the main challenge for large-scale MD is EC and WPC. New directions on MD should be raised. More rigorous investigations and focused directions on economical analysis of MD systems should be conducted. A unified standard method for analysis and calculations should be followed to determine WPC. For the benefit of MD process, one should be cautious when reporting simulated, non-realistic and non-contrasted WPC. (C) 2012 Elsevier B.V. All rights reserved.