In the application areas of polymer hydrogels, precise information on their molecular constitution as well as their elastic properties is required. Several interesting molecular features control the elastic properties of the hydrogels. In this chapter, we describe general properties of hydrogels formed by free-radical cross-linking copolymerization of vinyl/divinyl monomers in aqueous solutions. Special attention is paid to the relationships between the formation conditions of hydrogels and their properties such as swelling behaviour, elastic modulus, and spatial inhomogeneity. New developments achieved in the design of hydrogels with a good mechanical performance and a fast response rate is also presented.