How people learn a second language is a topic of long-standing human curiosity as well as prime educational and social importance. The mid twentieth century is the conventional starting point for recounting the history of the modern discipline of second language (L2) acquisition. However, although this chapter focuses on that interval, it recognizes that much older ideas and texts provide a context for understanding contemporary and near-contemporary treatment of the topic. To illustrate the scope of reflection on L2 acquisition, Chapter 1 narrates the historical backdrop to three themes, all of which are salient to the recent history of the field, while they have also appeared in various guises over the full-length history of Western reflection on language: (1) the role a learner’s first language plays in the acquisition of an L2; (2) capacities that are imputed to be inherent to learners, and which bear on the task of L2 acquisition; and (3) the function of the social environment in which learning takes place. These three themes provide a basis for appreciating continuities and discontinuities across the full history of L2 acquisition, a history that vastly predates the twentieth-century focus in this chapter.

As SLA diversifies in scope and scale, key questions remain open over the nature, development and use of a second (or third and additional) language. In this chapter, we evaluate the use of specific methods for different paradigms, while making the case for building inclusive and transdisciplinary connections to sustain the strength of the field. The chapter starts with research methodologies for investigating the development of L2 morpho-syntactic knowledge, as the traditional foundation of SLA. We distinguish between internal formal/cognitive questions around L2 linguistic representation and processing, typically focusing on group trends in quantitative findings, and external socially mediated research into L2 use, often focusing on exploratory individual experiences and motivation through qualitative data. We then present research into L2 phonology (L2 speech), complementing L2 morphosyntax in its focus on form, but often neglected in coverage of SLA research methodology. We finish by looking at implications for future SLA research, driven by greater interest in methodological rigor, replicability and open science, and the opportunities provided by the technological and social media revolution of recent years, e.g., using corpora, online/mobile platforms, and big data.