3In the first decade of life, children become bilingual in different language learning environments. Many children start learning two languages from birth (Bilingual First Language Acquisition). In early childhood hitherto monolingual children start hearing a second language through daycare or preschool (Early Second Language Acquisition). Yet other hitherto monolingual children in middle childhood may acquire a second language only after entering school (Second Language Acquisition). This Element explains how these different language learning settings dynamically affect bilingual children's language learning trajectories. All children eventually learn to speak the societal language, but they often do not learn to fluently speak their non-societal language and may even stop speaking it. Children's and families' harmonious bilingualism is threatened if bilingual children do not develop high proficiency in both languages. Educational institutions and parental conversational practices play a pivotal role in supporting harmonious bilingual development.