The world is becoming increasingly bilingual/multilingual, with a large number of children starting to learn languages other than their mother tongue in early childhood. This chapter provides an overview of our current understanding of child second language learners – successive bilinguals, whose exposure to an additional language begins around age 3. While most work on child L2 acquisition in the 1990s, until the mid 2000s was guided by various theoretical accounts and largely focused on morphosyntactic properties and developmental comparisons in different child L2 populations with adult L2 learners and those with developmental language disorders, much work reviewed in this chapter builds on recent theorizing and various phenomena that have significant advances in the field concerning child-internal and child-external factors. To this end, research over the last ten years forms the basis of this chapter, with a focus on how factors such as crosslinguistic influence, age effects, the amount and quality of input and the length of exposure determine young children’s acquisition processes. The examination of all these variables is argued to provide a more integrated and comprehensive approach to child L2 acquisition.