The use of energy transfer data and models in describing nonequilibrium polyatomic reaction systems is discussed with particular emphasis on the information needed for modeling vibrational energy transfer. In the discussion, it is pointed out that key areas of energy transfer knowledge are still lacking and the available experimental data are limited in scope and are of uneven quality. Despite these limitations, it is still possible to carry out meaningful simulations of chemical systems in which vibrational energy transfer is important. The vibrational energy master equation, which is the basis for modeling, and various experiments and calculations that provide the basis for practical energy transfer models and parametrizations are described. Two examples of gas phase reaction systems are presented in which vibrational energy transfer is important. The decomposition of norbornene shows how energy transfer parameters can be obtained from measurements of shock-induced chemical reactions, but even the best such experiments provide only limited information about energy transfer. Chemically activated 2-methylhexyl radicals illustrate the complex reactions of multiple isomers connected by multiple isomerization pathways and reacting according to multiple decomposition pathways.