This review gives a brief account of basic ideas underlying approaches to analysis and modeling of large-scale reaction systems. The emphasis is on model simplification and mechanism/dimension reduction via heuristic concepts and formal mathematical techniques. Among the key topics discussed are: top-down and bottom-up modeling approaches, graph/matrix representation of chemical reactions, mechanistic vs. pathways models, quantitative structure-reactivity relationships, mathematical reduction of dimensionality, high-fidelity surrogate models, continuum approximation, lumping of nonlinear kinetics, overall behavior/kinetics of many reactions, effect of pore diffusion, steady state multiplicity and stability. Some common features of dimension reduction methodologies are noted. Areas where further work will be valuable are identified.