Grammatical processing in a second language (L2) that is learned in adulthood has been shown to differ from processing in a first/native language (L1). Clahsen and Felser's (2006) landmark article provided the first comprehensive account of these differences. According to their shallow structure hypothesis (SSH), L2 learners, unlike L1 speakers, do not compute abstract, hierarchical representations during online sentence comprehension; instead, they rely on lexical, semantic, and pragmatic information to build ‘good enough’ representations. However, native-like processing is attainable – with sufficient L2 proficiency – for word-level processing and morphosyntactic feature processing between locally related words. Clahsen and Felser's article spurred a prolific volume of research over the last decade. Some findings support the SSH, whereas others favor the competing claim that L1/L2 differences result from capacity-based limitations.