The measurement, control and optimization of polymerization reactions are a challenge due to the complexity of the physico-chemical mechanisms and polymerization kinetics. Many important variables, which are related to polymer product properties, cannot be measured on-line or only at low sampling frequencies. Furthermore, polymer product properties are related to the molecular weight distribution, homo- or copolymer composition, sequence length, and branching. On-line NMR is a powerful technique for polymerization reaction and process monitoring with respect to the measurement of composition, molecular weight, particle size, and diffusion or viscosity properties. Flow rates, recovery times, and other parameters of the on-line NMR experiments must be adjusted for this purpose. Since it is generally prohibitive to use deuterated solvents in engineering applications, suitable techniques for solvent signal suppression are needed. A major task in engineering applications of NMR spectroscopy is the need for quantitative evaluation, therefore different approaches are discussed. In addition, the relation between NMR relaxation and molecular dynamics allow a detailed monitoring of and deep insight into the complex reaction mechanism in fluidic and solid-like reactions during processes. Finally, some of the trends in polymer industry are addressed, which may lead to future development in on-line measurement and reaction control.