FRP rebars present linear elastic behavior until failure. Their lack of plasticity can lead to sudden failure or significant loss of stiffness. Thus, establishing reliable predictions of its behavior is of utmost importance. The first approach to establish a failure criterion for FRP was based on an extension of isotropic criteria (von Mises for example) to include the orthotropic nature of these materials. The most general version of such criteria is the Tsai-Wu criterion. These types of criteria can predict FRP failure, however without taking into account different failure modes. Within the second generation of failure criteria, which are focused on combining the influence of different failure modes the most well-known are maximum stress/strain and Hashin-Rotem criteria. Another aspect of predicting damage in FRP materials is post-failure behavior. Mentioned criteria are used only to determine the end of the elastic behavior and after that, there is the need to characterize the mechanical response of the material to further loading—so the progression of the damage. Generally, some evolution laws need to be defined—starting from reducing material properties to lower, predefined values and coming to more complex approaches such as defining such laws based on fracture energy dissipation. This chapter also addresses the application of criteria to predict the failure of FRP rebars. Different potential approaches to defining post-failure behavior in such materials will be also analyzed. Finally, the potential usage of mentioned models in numerical simulations will be discussed.