This chapter discusses introduction, theoretical background and analysis of experimental results of polymer networks and gels. When long polymer molecules are chemically linked together to form a three-dimensional network, the resulting material exhibits a unique set of properties that have come to be referred to as "rubberlike". Among these are large deformation elasticity which has important consequences for mechanical behavior and resistance to solvent attack. As for the latter, when solvent molecules penetrate into the polymer it undergoes swelling rather than dissolution, and the diluted network is referred to as a chemically crosslinked gel. A survey of the thermodynamics and mechanics of crosslinked gels is presented. Subjects include a phenomenological description of crosslinked networks within a framework of finite elasticity theory and continuum thermodynamics. Particular emphasis is placed on the Valanis-Landel form of a strain energy density function. Several statistical mechanical models of rubber elasticity are also presented. Of particular usefulness are the affine and phantom network models, which are commonly used to derive information about molecular parameters of the gel from swelling or mechanical measurements. Techniques for using these models and the more modern Flory-Erman constrained junction model and its most recent modifications are described. The application of Scaling Theory to polymer gels is also considered. The main emphasis is to discuss the structure-property relationships of amorphous polymer networks and gels. This chapter focuses exclusively on the equilibrium properties of model networks in the dry and swollen states, and the relationships between macroscopically measurable physical quantities.