The ability to process the linguistic input in real time is crucial for successfully acquiring a language, and yet little is known about how language learners comprehend or produce language in real time. Against this background, we have conducted a detailed study of grammatical processing in language learners using experimental psycholinguistic techniques and comparing different populations (mature native speakers, child first language [L1] and adult second language [L2] learners) as well as different domains of language (morphology and syntax). This article presents an overview of the results from this project and of other previous studies, with the aim of explaining how grammatical processing in language learners differs from that of mature native speakers. For child L1 processing, we will argue for a continuity hypothesis claiming that the child's parsing mechanism is basically the same as that of mature speakers and does not change over time. Instead, empirical differences between child and mature speaker's processing can be explained by other factors such as the child's limited working memory capacity and by less efficient lexical retrieval. In nonnative (adult L2) language processing, some striking differences to native speakers were observed in the domain of sentence processing. Adult learners are guided by lexical-semantic cues during parsing in the same way as native speakers, but less so by syntactic information. We suggest that the observed L1/L2 differences can be explained by assuming that the syntactic representations adult L2 learners compute during comprehension are shallower and less detailed than those of native speakers.Assigning a grammatical structure to an input string presupposes knowledge of the combinatorial rules and grammatical constraints that apply in the language being processed. At the same time, however, successful grammar building presupposes the availability of appropriate mechanisms for processing the linguistic input (compare Chaudron, 1985;Fodor, 1998aFodor, , 1998bFodor, , 1999Valian, 1990). This apparent acquisition paradox poses a challenge for theories of first (L1) and second language (L2) acquisition that requires our existing knowledge of language learners' grammatical development to be supplemented by a detailed and systematic investigation of their grammatical processing routines. Although several decades' worth of psycholinguistic research has greatly increased our understanding of how mature readers and listeners process their native language in real time, psycholinguistically informed research into language learners' processing mechanisms

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