This book summarises the basic and fundamental aspects of hydrogel use in different industries, such as biotechnological, pharmaceutical, pollutant removal, circular economy, and food applications, among others. This book provides a useful resource for professionals and researchers actively involved in the development of synthetic and natural hydrogels, as well as postgraduate students and postdoctoral research fellows in materials sciences. The book is divided into six chapters contributed by experts in their research fields in order to provide a clear overview of each topic that describes "The application of hydrogels".