In the absence of defects or large second phase particles, fatigue initiation in engineering alloys is dominated by strain localization at specific microstructural features, in combination with high local stresses. Full-field crystal plasticity modelling provides, in principle, access to the stress and strain fields at the required scale, and therefore it has been used to make predictions of fatigue life and its sensitivity to material microstructure [1]. Although this approach appears to be successful, there is very limited direct validation of its predictions and therefore questions remain about its general applicability. Here,we discuss two attempts at validating crystal plasticity predictions of strain localization at the microstructural scale in structural engineering alloys. The first case is the strain localization at twist grain boundaries in Ti alloys, which is now widely accepted as both a site of enhanced strain localization and a fatigue crack initiation site [2]. Despite using a three-dimensional, experimentally determined microstructure, crystal plasticity modelling fails to predict any kind of deformation heterogeneity at this microstructural site. The second case is that of strain localization near an annealing twin at an abnormally large grain in a Ni alloy. Again, even when using a fully three-dimensional representation of the grain, crystal plasticity fails to predict the exceptional level of strain localization measured. These results demonstrate the limitations of using crystal plasticity for predicting strain localization at the microstructural scale in engineering alloys, and the need for continual direct experimental validation of modelling predictions when considering different alloy systems. [1] Castelluccio GM, Musinski WD, McDowell DL. Recent developments in assessing microstructure-sensitive early stage fatigue of polycrystals. Current Opinion in Solid State and Materials Science. 2014 Aug 1;18(4):180-7. [2] Liu C, Xu X, Sun T, Thomas R, da Fonseca JQ, Preuss M. Microstructural effects on fatigue crack initiation mechanisms in a near-alpha titanium alloy. Acta Materialia. 2023 Jul 1;253:118957.