So far, we have referred to single reactions. This chapter deals with the analysis in presence of multiple reactions (reaction networks). The chosen scenario is to provide, in a streamlined way, a general picture (in which the series and parallel schemes are presented, and the fundamental quantities such as yield and selectivity are defined) and a general approach (in which the methods of writing the material balances are illustrated, as a function of the reactor). All other considerations are left to the development of case studies that, although without the claim of generality, present the key characteristics of reaction networks. The chapter ends with considerations on the role of temperature.