The paper presents membrane distillation (MD), a separation process based on evaporation through pores of a hydrophobic membrane. Different configurations of the process were considered. Membrane properties, transport phenomena through the membranes, module designs have been discussed. Applications of MD in water and wastewater treatment as well as in food industry have been presented. The concentration of sulfuric acid solution obtained after extraction of apatite phosphogypsum by MD, in order to recover lanthane compounds has been presented. The possibility of recovery of HCl from metal pickling solutions by MD has been shown.