Generally, a chemically reactive system involves a mixture of different components subject to several simultaneous chemical reactions. When chemical reactions are modeled together with flow (and possibly temperature) variables, it is necessary to drop the hypothesis that the chemical composition in the flow domain is homogeneous. On the contrary, the composition will vary from point to point under the influence of the transport phenomena inherent to all dynamic situations. Chemical "species" will be diffused and transported by the flow, and chemical reactions will act as sources or sinks of the chemical species. Establishing a global model for this process of transport and reaction is described in this chapter.