Hydrogels are three dimensional (3D) cross-linked polymer networks capable of holding a large volume of water. The hydrophilic polymeric system sometimes exists as a colloidal gel inside water, i.e., dispersion medium. Hydrogels aim to mimic the 3D microenvironment of cells with the advantage of surpassing adverse gastrointestinal effects on the drug, therefore increasing patient compliance. This polymer-based hydrogel formulation has tunable properties such as porosity, tensile strength, drug loading capacity, and release kinetics that contribute towards better biocompatible hydrogel design. The monomeric units in hydrogels bind through physical and chemical forces such as hydrophobic interaction, hydrogen bonding, UV crosslinking, and many others. Albeit hydrogel is known for its water holding capacity and high biocompatibility, the cytotoxicity of hydrogel depends on the polymer selection. Deformable and injectable hydrogels that can alter its physical state in room and body temperature are in the research pipeline to avoid surgery for implantation. Further, environmental stimuli-responsive hydrogels like pH, temperature-sensitive hydrogels are evolving as ‘Smart drug delivery’ systems. This distinctive property of tunable hydrogel design and formulation finds its application in sustained and localized drug delivery. This chapter discusses the different classifications of the hydrogel, along with its crosslinking chemistry involved. We also have summarised various forms of hydrogel from lab scale to industrial level. Finally, this chapter also covers the synthesis, functionalization, tailoring mechanism of the hydrogel matrix, followed by in vitro, ex vivo, and in vivo characterization and drug loading/delivery efficiency. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2021.