 Hydrogels are hygrophilic, three-dimensional networks that absorb large quantities of water or biological fluids, have potential for biosensors, drug delivery vectors, and tissue engineering, respond to various stimuli, have two swelling mechanisms, gelation mechanism, mesh size calculation, natural and synthetic hydrogels discussed, and biomedical applications including cell culture, self-healing, and drug delivery. This article was authored by Chin Yuan Chai, Yang Jiao, and Xin Jiu and you.