Desalination is imperative to mitigate the global water scarcity as it produces drinking water from unpotable water. Currently, reverse osmosis membrane processes are widely used and account for 60% of desalination plants globally as they have lower energy requirements than other techniques, such as thermal desalination. Another promising alternative to desalination is membrane distillation (MD), which has been highlighted as one of the most promising and cost-effective desalination technologies over the last five decades. MD is a thermally driven desalination process that uses microporous and hydrophobic membranes through which only vapor can pass. Because non-volatile ions cannot pass through the membrane, MD theoretically achieves 100% salt rejection. In addition, MD is superior to other techniques as it is conducted at relatively low temperature and pressure, and is less sensitive to the feed concentration. MD is a desalination process that uses the vapor pressure difference between the feed and permeate as the driving force through the membranes. Over 2,800 scientific publications appeared in Web of Science as of September 2019 (over 400 just in 2019) describing the current state of development and potential future applications of MD. Although these publications provide excellent knowledge regarding MD, they are rather fragmented, and it is difficult to gain a complete overview of the basic principles and functions of membranes for MD configurations and their application to real plants. In this book, we introduce MD from the invention of this technique to the recent developments in membranes and processes. The membrane materials and configurations of MD processes are systematically discussed, along with an introduction to real pilot plants that have been installed and tested in the field, and an economic analysis of MD. The objective of this book is to provide a short, but reasonably comprehensive, introduction to MD to graduate students and persons with an engineering or natural science background, to gain a basic understanding of MD, and the associated materials, configurations, and applications, without studying a large number of different reference books.