In L2 acquisition, knowledge of derivational morphology and the combinatory properties of affixes and bases in the L2 has a positive effect on learners’ receptive and productive vocabulary. This chapter presents a learner-corpus study of affixation in the writing of EFL learners of diverse L1 backgrounds, focusing on various manifestations of cross-linguistic influence (CLI). The data suggest manifold effects of CLI such as overgeneralization and back-formation which predominantly affect non-native affixes and occur in all learner groups irrespective of the L1. However, there are also inter-group differences with Spanish and Italian learners showing comparatively strong interference effects in terms of foreignizing.