The present study investigated the relationship between implicit and explicit second-language (L2) processing in beginning L2 learners, and how cross-language similarity influences this relationship. The brain activity of native English speakers was recorded as they performed grammaticality judgments on Spanish sentences. The three types of agreement violations used were similar in the two languages ("Similar" type), different in the two ("Different" type), and unique to L2 ("Unique" type). After a baseline assessment, we improved participants' accuracy and then retested them on new and repeated items. Results showed that the explicit increase in accuracy was accompanied by a significant increase in brain sensitivity, as measured by the P600 ERP component. This effect was most pronounced for the Similar and Different types.Additionally, grammatical sensitivity was greater for repeated items in both measures of processing, and this was also modulated by cross-language similarity as well as the specific type of repetition. The obtained results are in line with an interface position on the relationship between implicit and explicit L2 processing and suggest a strong modulatory role of crosslanguage similarity in both types of processing.Keywords: Implicit/Explicit Processing; Second Language Processing; Cross-Language Similarity; ERPs; P600.iii Most adults who attempt to learn a foreign language can relate to the fact that it usually constitutes a highly effortful and oftentimes quite lengthy process. The ease with which one's first language (L1) was acquired is conspicuously absent and the developing second language (L2) has to now overcome the often detrimental influence of a highly ingrained L1 (e.g., MacWhinney, 2005). This, however, does not imply that a high level of proficiency and automaticity in the processing of an adult-learned L2 cannot be achieved. Some research has been conducted in the field of L2 learning examining the different ways in which beginning adult learners process their L2. Specifically, various studies have investigated the nature of this processing in terms of its implicit and explicit components, drawing a distinction between them.According to some authors (e.g., R. Hulstijn, 2005), explicit and implicit knowledge differ, among other things, in the extent to which one is consciously aware or intuitively aware of regular patterns in the information one possesses, and the extent to which one can or cannot verbalize such patterns, respectively. Furthermore, access to implicit knowledge is thought to occur automatically whereas access to explicit knowledge is thought to involve controlled processes (N. R. Ellis, 2005). The term explicit knowledge is often used interchangeably with the term declarative knowledge. Thus, two important distinctions between implicit and explicit knowledge of an L2 are the level of awareness as well as the amount of effort involved in accessing relevant linguistic rules or patterns.The goal of the present study was to investigate the relationship between implic...