After years of research focusing on universal regularities in child language, the study of variation has enriched our understanding of language development. Variation stems from exogenous and endogenous factors that can all influence the development of communicative capacities. This volume focuses on three main sources of variation considered from a large multimodal perspective that includes speech, gesture, and signs. First, research suggests that development partly varies with linguistic and/or cultural environments. A second source of variation stems from the input to which children are exposed across contexts. Finally, learners themselves differ along many dimensions, such as cognitive maturity, exposure to language(s), relative reliance on the visuo-gestural modality, and language impairments. The discussion highlights the need for more research on inter- and intra-individual variation within comparative perspectives allying complementary methodologies.