Second language acquisition (SLA) is a relatively new field of enquiry. Before the late 1960s, educators did write about L2 learning, but very much as an adjunct of language teaching pedagogy, underpinned by behaviourism, the then-dominant learning theory in psychology. In this view, the task facing learners of foreign languages was to rote-learn and practise the grammatical patterns and vocabulary of the language to be learnt, in order to form new ‘habits’, that is to create new stimulus–response pairings which would become stronger with reinforcement. In order for the ‘old habits’ of the L1 not to interfere with this process by being ‘copied’, or transferred, into the L2, researchers embarked on thorough descriptions of pairs of languages to be learnt, in order to identify areas that are different and would thus be difficult.

The focus of enquiry at the time was therefore very much the description of L1–L2 pairings, and little attention was being paid to what foreign language learners actually did with the input they received, or to their actual productions in the L2. This changed in the second half of the 1960s, primarily as a result of the Chomskyan ‘revolution’ in the field of L1 acquisition. L1 children were shown to be highly creative in their acquisition of language, rather than mere imitators of the language around them. In the context of L2 acquisition, researchers started focusing on what learners actually produced for the first time, especially their errors, drawing the conclusion that much of their productions cannot be traced back to their L1, nor the L2 they are exposed to. Additionally, research found that what is different in two languages is not necessarily difficult for learners, and what is similar not necessarily easy.

If I lingered a while on these very early stages, it is because this very simple fact – namely that in order to understand SLA we need to investigate what learners actually do and produce, as well as the context in which they learn, rather than merely focus on the description of source and target languages – led to a major shift in SLA theorising. Redefined in this way, the field needed to turn to a wide range of neighbouring disciplines in order to do justice to its multifaceted nature. Descriptive linguistics and behaviourism were no longer the only disciplines relevant to this endeavour, and researchers started drawing on theoretical frameworks having their origins in psychology (e.g. processing, individual differences), theoretical linguistics (syntax, lexis, semantics, discourse, pragmatics, phonology), education, sociolinguistics, L1 acquisition, sociocultural theory, neurolinguistics and others. This led to a myriad of theoretical approaches, sometimes complementary, sometimes incompatible.

The following timeline traces this journey. Its emphasis is therefore historical, prioritising works which were influential at the start of a new line of enquiry, and focusing on theories which have had a lasting impact on SLA research and given rise to many studies. Very recent theorising is therefore not given the same prominence as earlier research, as it is more difficult to say how influential it will eventually be in shaping the field. The focus is on the processes involved in acquisition, rather than on the teaching of foreign languages, as these two research fields have relatively little overlap. The treatment of the various theoretical approaches is inevitably oversimplified and highly selective, but it is hoped that the reader will get a good overview of the development of this highly complex and multifaceted field of research. The works cited are mostly influential theoretical pieces, but also sometimes empirical studies which started a major new line of SLA theorising. Readers interested in more detailed treatments of the subject may wish to consult, for example, Mitchell & Myles (Reference Mitchell and Myles2004) or R. Ellis (Reference Ellis2008).