This lecture covers the nonlinear behavior of geomaterials, including irreversibilities and hysteresis, with a focus on the Cam-Clay model. It discusses the limitations of usual simplifications in geomechanics, such as elasticity and perfect plasticity, and introduces the generalized Hooke's law. The course also explores rigid-perfectly plastic materials, their causes of plasticity, and the laws studied, such as Von Mises and Mohr-Coulomb. Various figures and equations are used to illustrate the concepts.