This chapter centers on the major descriptive findings of L2 research, focusing on ordered and systematic development. We review and discuss such things as morpheme orders, developmental stages/sequences, unmarked before marked, and U-shaped development, among others. We also review the evidence for L1 influence on ordered development. We touch on the nature of internal (e.g., Universal Grammar, general learning mechanisms) and external constraints (e.g., quantity and quality of input and interaction with that input, frequency) as underlying factors in ordered development. We also briefly touch upon variability during staged development.

In this chapter we review the qualitative difference between explicit knowledge and implicit knowledge (underlying mental representation). The chapter focuses on whether instruction affects the latter. We review the accepted finding that instruction does not affect ordered development. We also review the issue of whether instruction affects rate of development and ultimate attainment. We review important variables in the research on instructed acquisition including type of knowledge measured, the nature of assessments used in the research, and short-term vs. long-term studies, among others.

This chapter lays the foundation for how the field of second language acquisition arose. We briefly review the pioneering work in the late 1950s and 1960s in first language acquisition (e.g., Berko Gleason, Brown, Klima & Bellugi). We also review the generative revolution in linguistics and how it laid the groundwork for the idea of constrained language acquisition. We then review the seminal articles by S. Pit Corder (1967) and Larry Selinker (1972) that posited the major questions in second language acquisition, and end with the pioneering work that mirrored research in first language acquisition (e.g., Dulay & Burt, Krashen, Wode). We end the chapter with the major question that launched second language acquisition research in the early 1970s: Are L1 and L2 acquisition similar or different?

This chapter defines what kind of input contains the data necessary for acquisition (communicatively embedded input) and focuses on its fundamental role in acquisition. Subsequently, we review the claims on the role of output and interaction, focusing on these major issues: Comprehensible output is necessary for acquisition; comprehensible output is beneficial for acquisition; comprehensible output does little to nothing for acquisition. We also discuss the nature of interaction more generally, focusing on whether interaction affects the acquisition of formal features of language.

In this chapter we touch on the idea of inter-learner variability in outcome (i.e., how far learners get) as well as rate of acquisition among different learners. We then link these issues to the idea of individual differences as explanatory factors. We focus on the most studied: motivation, aptitude, and working memory.

This chapter covers three different ideas about nativelikeness in L2 acquisition. The first is that learners can become nativelike in all domains of language and language processing. The opposite idea we will cover is that learners cannot become nativelike in any area of L2 acquisition. The final idea we will treat is that learners can become nativelike in some domains of language but not others. We discuss what “nativelike” means and what kinds of measures are used to assess learner knowledge and ability. We also review key hypotheses and constructs such as the Fundamental Difference Hypothesis, the Shallow Structure Hypothesis, Full Transfer/Full Access, the critical period, and others.

In the epilogue, we reflect on some of the major themes linked to the questions driving research in second language acquisition and return to the question that started the field in the 1970s: Are L1 and L2 acquisition similar or different?

In this chapter we address the question of whether or not language acquisition is largely implicit in nature. After reviewing key constructs (e.g., explicit and implicit knowledge, explicit and implicit processing/learning, intentional and unintentional learning), we discuss the major positions currently under scrutiny in the field: (1) Explicit learning is necessary; (2) explicit learning is beneficial; (3) explicit learning does little to nothing (i.e., acquisition is largely if not exclusively implicit in nature). A key issue in this chapter is how one defines “language” and how one construes “input processing.” We will review how definitions of these constructs color the researcher’s perspective on the issues.

In this chapter we review the competing perspectives on the role of language transfer, namely, the role that a first language plays in the acquisition of a second language. Do learners begin with the L1 and transfer all properties and processes into L2 acquisition? Or is transfer partial and selective at the outset? Or do L2 learners not transfer any aspects of the L1 and begin with universal properties of language and universal processes for acquiring language? As such, we review such key hypotheses as Full Access, Full Transfer/Full Access, and important constructs such as minimal trees, input processing, and processability. We also touch on the notion of third and subsequent language acquisition and competing perspectives on what languages get transferred if any (and how).