This chapter provides a brief introduction to language acquisition research. Our survey focuses on child and adult language learning, so we cover both the acquisition of native language (L1) or languages, and of additional languages (L2, L3, etc.). In doing so, we will first describe the methods typically used to investigate language learning (Section 9.2) before introducing a range of topics that have been widely studied over the past 50 years. We describe the mechanisms by which language is acquired, explaining these in terms of the age-old debate about the role of ‘nature’ and ‘nurture’ (Section 9.9.3). In the following two sections we then review factors that affect the course of language development. We begin by focusing on factors that are generally associated with the nurture side of language acquisition (Section 9.4). Here, we are going to look at different properties of the linguistic input that infants and children receive from their caregivers and peers and how they process this input to learn the phonology, lexicon and grammar of the target language. We will also look at the role of interaction and instruction in language acquisition. Next, we will explore factors that are associated with the nature side of the debate (Section 9.5). We discuss how learning is affected by our genetic endowment, our age, the languages we have acquired previously, and by cognitive and socio-cognitive variables. We have decided to keep this survey broad, going for breadth of coverage rather than depth, in the hope that you will find a topic that sparks your interest. You can then further explore this exciting research by consulting helpful textbooks such as Brooks and Kempe (2002), Clark (2013), DeHouwer (2021) or Ortega (2009).