Gels are physically or chemically crosslinked polymer networks swollen with solvents. Specifically, gels swollen in water are known as hydrogels and find extensive applications such as foods like jelly and agar, hygiene products like diapers, and medical devices like soft contact lenses. They possess unique properties, making them versatile materials applicable in fields ranging from medicine to environmental and energy-related contexts. The ubiquity of gel applications finds its foundation in their extraordinary properties, notably encompassing water absorption, elasticity, separability, adsorption, permeability, optical characteristics, and biocompatibility. For example, the water-absorbing capacity proves crucial in products like diapers, while the transparency, gas permeability, and biocompatibility of soft contact lenses hinge upon these unique gel attributes. These properties stem from the affinity between the polymer network of gels and the solvent, as well as the crosslinking structure, creating an open system allowing the movement of substances and energy within and outside the gel. Furthermore, the discovery of the volume phase transition in gels triggered a significant advancement in the physics and chemistry of gels. Recently, to overcome the low mechanical properties of gels, some researchers have adeptly engineered network structures, strategically affording the preparation of mechanically robust gels. From a foundational standpoint, gels emerge as captivating subjects in polymer science and technology, spawning diverse applied research endeavors owing to unique properties. This chapter introduces the captivating capabilities of gels and highlights their fundamentals and applications. The chapter also provides an overview of important studies focusing on the distinctive properties of gels.