This lecture covers the development of constitutive models based on physical principles, focusing on the high strain rate behavior of materials and the evolving microstructure during deformation. It discusses the obstacles dislocations face, such as Peierls-Nabarro forces, and the role of thermal energy in dislocation motion. The lecture also explores the rate dependency of plastic response, the interaction of dislocations with defects, and the constitutive equations derived from activation barriers. Additionally, it delves into the strain rate sensitivity of materials and the evolving microstructure's impact on stress-strain behavior.