The definition of 'important' depends on the needs of the modeler: which species can be safely ignored? What is the modeler's error tolerance? It also depends on the reaction conditions: ethanol changes from an inert solvent into a major reactant at high temperatures (e.g. in the process for making ethylene from biomass). Often there is a core set of species and reactions that are currently considered to be important, and then a much larger set of other species and reactions which could conceivably be important, as illustrated in Fig. for the case of propene pyrolysis. In Fig. the black species and solidarrow reactions are currently thought to be important at the reaction conditions of interest, while it is currently unknown if the dashed reactions and gray species are important. And of course there are thousands of other reactions and species that could conceivably be important, but which are not currently under consideration, these are not shown in the figure . A systematic method is needed, both to propose candidate reactions and species, and to decide which are actually important.