In this work the structure, formation and thermodynamic properties of polymer networks are investigated and characterized by different NMR techniques and swelling experiments. A focus was on the determination of crosslink densities and the influence of the network structure (i.e., defects) on the evaluation of swelling experiments according to the Flory-Rehner theory. The validity and applicability of the Flory-Rehner theory is examined by temperature-dependent swelling experiments with solvents of varying quality and NMR experiments on equilibrium swollen networks. The local stretching respectively deformation of polymer networks with different topologies is investigated at low, medium and high degrees of swelling by 1H double-quantum low-field NMR. For this method a standardized reliable evaluation procedure is presented. Finally, the reaction kinetics and the formation of the elastically active polymer network in solution are investigated by high and low-field NMR.