Hydrogels are 3-D (three-dimensional) polymer networks that can be natural or synthetic. After encountering water, do not dissolve but swell. Hydrogels swell in different conditions, such as temperature, pH, and ionic strength, and show other behaviors. Two initiators are used to prepare hydrogels: chemical and radiation initiation. Today, hydrogels have many applications, especially in medical and biomedical science. Vinyl hydrogels are the most applicable ones by worldwide 'polymer chemists'. Allylic monomers cannot be easily polymerized by the free radical polymerization method. The achieved polymers have low molecular weight and cannot convert to hydrogels typically. Inter-penetrating polymer network (IPN) hydrogels can be used in the wastewater remediation industry. Amphiphilic and amphoteric semi-(IPN)s are asserted in the metal adsorption and water purification industry.