Through equilibrium we relate applied forces to stress and through kinematics we relate displacement to strain. To close the loop and form a complete theory, we must relate stress to strain. A relationship between stress and strain is called a constitutive model. While the ultimate goal of constitutive theory is to write a universal law governing material behavior, multiple practical considerations make it infeasible to describe all materials with one law. Hence, unlike equilibrium and kinematics, constitutive relationships are empirical. In this chapter, we explore one of the most common constitutive models of linear elastic response.