A dynamical system is calledchaoticif small changes to its initial conditions can create large changes in its behavior. By analogy, we call a dynamical systemstructurally chaoticif small changes to the equations describing the evolution of the system produce large changes in its behavior. Although there are many definitions of “chaos,” there are few mathematically precise candidate definitions of “structural chaos.” I propose a definition, and I explain two new theorems that show that a set of models is structurally chaotic if it contains a chaotic function. I conclude by discussing the relationship between structural chaos and structural stability.