Much of our knowledge about how people learn additional languages was forged during the 1980s and 1990s under a cognitive-interactionist perspective on L2 learning. Cognitive-interactionism is associated with the work in developmental psychology by Jean Piaget (e.g. 1974) and refers to the position that multiple internal (cognitive) and external (environmental) factors reciprocally interact (hence the word ‘interactionist’) and together affect the observed processes and outcomes of a phenomenon – in this case, additional language learning. It is noteworthy that internal cognition is assumed to be the locus of learning (hence the word ‘cognitive’ in the term) and that a clear separation between cognitive-internal and social-external worlds is presupposed, since how the two interact is the object of inquiry. More recent SLA research inﬂuenced by a number of related sociocultural perspectives has challenged the assumptions of cognitiveinteractionism, and we will discuss these other ways of conceiving of language learning in Chapter 10. In the present chapter and the following one, however, we will examine ﬁndings about the environment and cognition that have been gleaned using a cognitive-interactionist prism.