We present in this chapter an overview and a state of the art on the Membrane Distillation (MD) process. Such a process is a new, rapidly increasing membrane technology, which can be used for desalination and also for recycling. It is a separation process that involves transport of vapor through porous hydrophobic membranes. The principle, the main applications and the benefits of this separation technique are outlined. Approaches to improve the process performance are also discussed.