Substances that absorb significant quantities of water are called gels or hydrogels. Naturally occurring materials with these properties play a very important role in all forms of life. In this Handbook, the biomedical applications of hydrogels are addressed by experts in the field from around the world. The phenomenal properties of hydrogels continue to stimulate scientists to seek new insights into the development of novel biomaterials and bioapplications. Composed of three-dimensional polymer networks, hydrogels can absorb large quantities of water. Consequently, they are soft, pliable, wet materials with a wide range of potential biomedical applications. Hydrogels are currently widely used in bioapplications and play a crucial role in modern strategies to remedy malfunctions in and injuries to living systems.