Intragranular localization of plastic slip plays a key role in the deformation and fracture mechanisms of metallic materials. Accurately simulate the formation of individual slip localization bands within grains, and their impact on the behavior of a polycrystalline material is a twofold challenge. First, these bands are discrete objects with characteristic length scales. Hence standard crystal plasticity models fail to predict them, which calls for the development of more advanced models. Second, simulating polycrystalline unit cells with non-linear behaviors, with a mesh that is fine enough to resolve localization bands comes with a very large computational cost. This talk will present a promising framework to address this challenge. It relies on the association of softening plastic flow equations and a gradient plasticity formulation [1], and on the use of FFT-based mechanical solvers, which have emerged as a very effective tool for polycrystalline simulations [2]. The softening formulation yields instabilities forming of localization bands, and the gradient formulation introduces length scales and physical processes that allow simulations to capture important features of observed slip band networks in polycrystalline materials. The simulations involved in this work have been achieved thanks to the massively parallel FFT-based solver AMITEX_FFTP (available at https://amitexfftp.github.io/AMITEX/). [1] Marano, A., Gélébart, L., & Forest, S. (2021). FFT-based simulations of slip and kink bands formation in 3D polycrystals: influence of strain gradient crystal plasticity. Journal of the Mechanics and Physics of Solids, 149, 104295. [2] Lebensohn, R. A., & Rollett, A. D. (2020). Spectral methods for full-field micromechanical modelling of polycrystalline materials. Computational Materials Science, 173, 109336.