Some binding processes are cooperative, rather than independent. One molecule binds to a target; then a second molecule binds with a different affinity. In such cases, the independent-site model of Chapter 27 is too simple. Coupled binding is the core operating principle of molecular machines in biology, described in the next chapter. Coupled binding also explains self-assembly processes, such as the formation of soap micelles and lipid bilayers. Here, we introduce the methodology for modeling cooperative binding, called binding polynomials.