This chapter considers routes to network formation: cross-linking of preformed chains and direct polymerization including multifunctional monomers. It discusses the other general approach to network formation or gelation, using polymerization of multifunctional monomers. The chapter describes successively elastic deformations, thermodynamics of elasticity, the “ideal” molecular description of rubber elasticity, and then extensions to the idealized theory. The chemical potential gradient will drive solvent into the piece of network, but the elasticity of the network will resist unlimited deformation. A simple and very qualitative way to understand the origin of such a term is that there is an additional entropy gain for the placement of the end of each strand in space, as the total volume of the network plus solvent increases. A constant force is applied to an ideal elastomer, assumed to be a perfect network.