The proposed research is designed to address the question of why some adults are better than others at learning a second language (L2). The proposal focuses on the cognitive underpinnings of L2 processing, and aims to expand on previous research in two ways: 1) by exploring a larger range of cognitive variables than has previously been investigated, and 2) by examining both lexical and syntactic processing, and the relationship between them. The overarching hypothesis is that a large source of the variability among individuals is rooted in the ability to resolve the competition that arises as a result of two languages being activated. The proposal is comprised of four experiments, each of which includes a battery of tasks designed to measure individual differences in first language (L1) cognitive processing. Tasks will include working memory span, Stroop color naming, L1 grammaticality judgment, and task switching. The first experiment will assess the degree to which L2 learners rely on their L1 to translate words and name pictures, and to detect grammatical errors in L2 sentences. In the second experiment, participants will learn novel words in association with pictures of either familiar or novel objects, in order to assess whether L2 words can be learned more easily when there are no competing L1 words. Experiment 3 will use a bilingual picture-word interference task and an agent identification task to compare the effects of competition at the lexical and syntactic levels. Experiment 4 will consist of a variety of bilingual switching tasks that will examine the ability of L2 learners to rapidly control the relative levels of activation of each language.