There is a growing need for better membranes in several emerging application fields especially those related to energy conversion and storage as well as to water treatment and recycling. Processability, is an important functional property, often ignored, especially in the early discovery phase for new materials, but it should be one of the most important properties, that needs to be considered in the development of better membrane materials. Useful membrane materials have to be capable of being formed into thin membranes, in particular for membrane gas separation, water treatment and desalination, and then packaged, into large area membrane modules. All gas separation membranes that are in current commercial use are based on polymers, which are solution-processable. This book intends to deal with composite, in most cases hybrid polymer-based membranes for three separate application fields: energy conversion, energy storage and water treatment and recovery. Each chapter will explain clearly the various membrane processes then go on to discuss in detail the corresponding advanced membranes used. The logic that lies behind this is that you have to understand the process in order to develop new high-performance membranes. By taking this approach, the author aims to overcome the disconnection that currently exists between membrane materials scientists and industrial process engineers.