People greatly differ in how fast, how well and by what means they learn an additional language. The variability in rates, outcomes and processes can be strikingly large, particularly for people who begin learning an L2 later in life. For example, in Chapter 2 you read about Julie (Ioup et al., 1994), who was remembered by friends and family to be as good as native speakers after only two and a half years of ‘picking up’ Arabic from living immersed in the L2 environment; but you also read about Wes (Schmidt, 1983) in Chapter 4, who appeared to make only slow progress in his acquisition of English even after three years of positive engagement in the L2 environment. In light of this evidence, SLA researchers ask themselves: Is there something in learners’ cognitive abilities, their motivations and their personal predispositions that could help explain such a wide variation? This question guides the study of what is known as individual differences in L2 learning, an area of SLA that draws on theories and methods from the neighbouring fields of cognitive, social and crosscultural psychology.