This chapter examines the role of natural configurations, meaning stress-free configurations. If a solid body is free of external body forces and external tractions, then it may or may not be stress free. If such an externally released body is not stress free then it is residually stressed. In such a case the residual stress can be revealed by various cutting operations whereupon the resulting spring-back upon release characterizes the nature of the residual stress. Residual stress can arise by a variety of mechanisms, including natural configuration mismatch across an interface, swelling, and growth. These are examined in the context of relatively simple examples, using both a compressible and an incompressible material framework. Lastly, constitutive models for complex time-evolving microstructures are examined in the context of the theory of multiple natural configurations. This has been used to model the time evolution of both interpenetrating polymer networks and remodeling in collagenous soft tissue.