Who is the language learner represented in language acquisition research? Despite the prevalence and diversity of multilingual childhoods, theories of early language development remain grounded in monolinguals from Euro-American contexts, while multilingual research has concentrated on limited populations/settings. This review examines multilingual language development during the first three years across Asia and Africa, attending to diversity within and between regions. Research on input, speech perception, word recognition, and vocabulary indicate that multilingual infants flexibly adapt to linguistic ecologies, but vary in developmental pathways. We argue that multilingualism should be conceptualized not as a categorical characteristic, but as a multidimensional ecology structured by language combinations, caregiving arrangements, social functions, socioeconomic conditions, and colonial histories. We conclude with a research agenda that broadens empirical and linguistic coverage, improves measurement of multilingual experiences, and centers community-informed priorities. A globally adequate science of language development requires building theory from children’s diverse multilingual realities.