Clahsen and Felser (CF) review ground-breaking work comparing selected types of language processing in monolingual children and adults, on the one hand, and in monolingual first language (L1) adults and adult second language (L2) learners, on the other. They argue that children behave essentially like adults, but that adult L2 learners, even high-proficiency ones, do not. Thus, there is a principled difference to be made among types of learners; there is continuity of mechanism and process to be observed in monolingual development but L2 acquisition exhibits certain fundamental differences. In particular, L2 learners construct shallow syntactic structures (essentially failing to compute trace chains) when processing long-distance filler-gap dependencies. According to the shallow structure hypothesis (SSH), learners immediately interpret incoming words in a minimal semantic representation by assigning thematic roles to argument expressions and associating modifiers to their hosts. They are not mapping detailed and complete syntactic representations onto semantic representations.