Researchers have focused for decades on how young children learn individual words. However, they have paid less attention to how children organize their word knowledge into the network of representations that underlies our ability to retrieve the right words efficiently and flexibly when we need them. Although methodological limits have made it difficult to study the development of lexical–semantic networks, in recent work with new paradigms, infants and toddlers have shown that they organize their vocabulary systematically via both categorical and associative relations from around age 2. Combined with work demonstrating that lexical–semantic relations affect early comprehension, production, and learning, this emerging area of research suggests that scientists and practitioners would benefit from more comprehensive theories of language learning that include not only vocabulary development, but also the development of lexical–semantic networks.