In recent years, many researchers have devoted considerable attention to the development of membrane science and technology. Different important types of membranes, such as those for: nanofiltration, ultrafiltration, microfiltration, separation of gases and inorganic membranes, facilitated or liquid membranes, catalytic and conducting membranes, and their applications and processes, such as wastewater purification and bio-processing have been developed. In fact, almost 40 % of the sales from membrane production market are for purifying wastewaters. Hydrogel membranes fulfil many of the important conditions for most of above-mentioned application fields. Therefore, the authors have focused this book on the applications of PVA-based membranes in areas such as for separation membranar processes, fuel cells, sensors, biochemical/medical applications, catalyst or PVA derivatives membranes as gas and vapour barriers.