Understanding the mechanisms learners use to process target language input is crucial to developing a complete model of both first language (L1) and second language (L2) acquisition. If adult L2 learners are found to process the target language with mechanisms that differ from those used by child L1 learners and adult native speakers, what implications might this have for the developing grammar? Clahsen and Felser review evidence that appears to point to such differences, generalizing their findings under a shallow structure hypothesis about how adult learners process input in L2.