The central theme of this book is the mechanics of elastic deformable solids, built on the three-legged stool of equilibrium, kinematics, and constitutive behavior of materials. Throughout the book linearize kinematics and constitutive models have been invoked to solve specific problems. The linear equations allow us to see how forces flow through solid bodies and what deformations accrue in the process. However, the equations do not allow us to explore the strength of the solid body or to understand the consequences of linearizing the equations. This chapter is a very brief introduction to the strength and stability of solids, addressing the capacity of systems, not just their linear response.