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This paper focuses on the interactions between the languages in the multilingual mind with reference to studies on cross-linguistic influence, dominance shift and, particularly, language attrition. We first address first language attrition by providing a brief state of the art with respect to three main questions: why does attrition arise, how does it happen and what kind of structures are mainly concerned by attrition. Psycholinguistic factors influencing interactions with the mental lexicon are specifically discussed in order to explain why some words are more vulnerable to semantic extensions than others. We then describe some main results of studies on second and foreign language attrition with focus on the role of extralinguistic and linguistic factors.