Cognition refers to how information is processed and learned by the human mind (the term comes from the Latin verb cognoscere, ‘to get to know’). SLA researchers interested in cognition study what it takes to ‘get to know’ an additional language well enough to use it fluently in comprehension and production. We are far from a satisfactory understanding of second language as a form of cognition, however. This is because our capacities to investigate the relevant questions are shaped by the pace at which new theories and methods to inspect the workings of human minds and brains become available (typically in neighbouring disciplines) and the rate at which SLA researchers become conversant in them. In this chapter, more than in any other, I will make frequent reference to relevant L1 research and point at areas where future attention by SLA researchers will be needed.