A hydrogel is a three-dimensional (3D) network of hydrophilic polymers with the potential to swell in water and hold a large amount of water. These polymers can maintain the structure due to chemical or physical cross-linking of individual polymer chains. Hydrogels possess a degree of flexibility very similar to natural tissue due to their significant water content. The presence of hydrophilic groups such as -NH2, -COOH, -OH, -CONH2, - CONH -, and -SO3H are responsible for the hydrophilicity of the network. Hydrogels undergo a significant volume phase transition or gel-sol phase transition as a reaction to certain physical and chemical stimuli. The examples of physical stimuli include temperature, electric and magnetic fields, solvent composition, light intensity, and pressure. The chemical or biochemical stimuli include pH, ions and specific chemical compositions. Hydrogels are capable of returning to their initial state after a reaction as soon as the trigger is removed, therefore such transitions are reversible. The topics included in this book on the characterization and applications of hydrogels are of utmost significance and have applications in multiple industries. The readers would gain knowledge that would broaden their perspective about hydrogels through it.