In recent years, thermosetting resins have been used in various applications because of the ease in controlling viscosity before hardening, as well as their superior handling, compared with thermoplastic resins. However, the various types of monomers, resins, and methods contributing to hardening in diverse applications have increased the complexity of the resin systems. To obtain a hardened product with physical properties close to its designated purpose, it is necessary to understand the properties of the monomers and resins that contribute to hardening, as well as their hardening mechanisms, and to perform appropriate curing. The crosslinking (hardening) of alkyl vinyl groups explained in this section has been increasingly utilized recently for fabricating electronic materials with low dielectric properties and insulation, as well as for preparing rubber modifiers with enhanced mechanical strength, water resistance, and oil resistance. In this presentation, we explain thermal hardening, or curing, of monomers and resins containing an alkyl vinyl group and provide some examples.