This issue was designed to include a wide range of research on children's second language learning. Here we provide a short overview of each of the articles contained in this issue, many of which bring up novel ideas and topics, as well as new takes on familiar themes that sometimes challenge prior conceptions and, ideally, inspire new understandings of child language acquisition, and policies, and practices in instructed settings. The 15 articles in this issue are based in instructed and naturalistic settings and include reviews and experimental work, and collectively represent learners between 5 to 18 years old. The language backgrounds include Mandarin (first language [L1]), Arabic (L1), Basque (L1), Cantonese (L1), English (second language [L2]), Hebrew (L1, L2), Spanish (L1, L2), and Thai (L1). Topics include the uniqueness of child second language acquisition (SLA); learning in majority language classrooms; best practices in bilingual schooling, cognition, and SLA in younger learners; testing and assessment relating to age and language choice; and methodological contributions that arise from the particular challenges of researching child second language development in instructed and naturalistic settings.