Multiscale modeling, which merges the worlds of macro- and micromechanics, is establishing itself as a viable alternative to experimental procedures in the characterization of the mechanical behavior of complex materials. Advanced composite materials are a perfect field for the application of such modeling concepts. This thesis focuses on failure mechanics of fiber reinforced composites and addresses the modeling of failure processes at both micro- and macro-scales. First, a novel damage-plasticity model is developed and implemented within finite element software Abaqus as a user defined element. It is verified that the model gives mesh objective results, and the model is calibrated with experimental stress-strain curves from the literature. Representative volume elements (RVEs) based micro-mechanical models are constructed where damage-plasticity model and cohesive surfaces are employed to capture failure in matrix and matrix-fiber interface, respectively. A sufficiently large number of RVE analysis results are used to generate discrete failure envelopes. These failure envelopes are compared with continuous ones resulting from Puck’s criteria. Furthermore, the influence of microstructural imperfections is investigated systematically, and an extended version of Puck’s criteria is assessed from a micro-mechanical perspective as well. In the last part of the thesis, a macroscopic model is proposed which combines Puck’s criteria with localizing implicit gradient damage model. It is shown that the model provides consistent results such that the failure angle obtained at material point and the orientation of the emerging macroscopic damage band match provided that sufficiently small internal length scale parameter is used.