This lecture covers the fundamental concepts of plasticity in geomechanics, focusing on the constitutive modelling of geomaterials. It discusses the principles of plasticity, including yield criteria, Von Mises, Drucker-Prager, and Mohr-Coulomb models. The lecture explores the application of elastic-perfectly plastic models in analyzing gravity-fissured dams and underground powerhouses. It also delves into the graphical representations of yield criteria and the triaxial CTC tests for different models. The presentation concludes by emphasizing the importance of plasticity in describing the non-linear behavior of geomaterials and the practical applications of elastic-perfectly plastic models.