Membrane distillation (MD) is a non-isothermal membrane technology that is increasingly gaining prominence compared to classical distillation. The work presents the development of this separation technology, the mechanism of the MD process, types of MD membranes, MD modules and practical application. The pressure difference is the main driving force of MD, therefore four main modules of this separation method were developed: direct contact membrane distillation (DCMD), sweeping gas membrane distillation (SGMD), air gap membrane distillation (AGMD) and vacuum membrane distillation (VMD). For the MD process itself, hydrophobic microporous membranes are used, usually made of poly(tetrafluoroethylene) (PTFE), polypropylene (PP) and poly(vinylidene-fluoride) (PVDF) in order to prevent wetting of the membrane. The work also explains the phenomena of membrane wetting, penetration pressure and vapor-liquid equilibrium, as well as the phenomena of mass and heat transfer through the membrane. Applications of MD are the most common in desalination of sea and brackish water, separation of molecules such as ammonia and aromatic compounds as well as heavy metal ions. MD also found its application in the concentration of aqueous solutions. Due to the easy integration of the MD system into other separation plants, it is possible to achieve satisfactory results such as the reduction of concentrate.