In the previous chapter, we learned that an important aspect of the dynamical behavior of regulatory systems is determined by the structure of the regulatory network. In this and the next chapter, we will introduce structural theory for another class of networks in the life sciences, reaction networks. We will show that various aspects of the behavior of a reaction system are determined from the structure of the reaction network. In this chapter, we will focus on the response of the system when the parameters change, for example, the changes in the concentration of chemical substances when the amount or activity of the enzyme changes. The molecular species that show concentration changes to a change in enzyme activity are determined solely from the structure of the network.