This chapter reviews the evidence accounting for individual differences in second language (L2) processing. It presents an overview of how certain factors relating to L2 input variability affect individual differences and discusses current knowledge of the critical period hypothesis, a core debate in explaining individual differences in non-native language processing. The chapter presents an overview of brain imaging studies where monolinguals and bilinguals have been compared, and provides a general framework to better integrate the results, highlighting the differences in the neural substrate that the learning of a second phonological system induces in the brain. It reviews variability induced by endogenous factors, that is, what is commonly known as aptitude in the acquired language. The chapter presents studies where a non-native phoneme contrast has been learned through laboratory training, and studies investigating individual differences resulting from learning in natural contexts. It concludes by summarizing the main findings and pointing to potential new lines of research.