The studies included in this dissertation explore lexical-semantic development within early toddlerhood across monolingual and bilingual language learners. That is, when and how do children map words with their respective meanings when they are learning one or two languages? Chapter 1 presents a brief overview of the seminal questions that guided this dissertation research. Chapter 2 presents a comprehensive review of the empirical literature examining lexical-semantic development in young monolingual and bilingual children followed by a discussion of theoretical models and their ability to account for the available evidence. Importantly, the chapter ends drawing attention to several gaps in our understanding of bilingual lexical development that are the foci of Chapters 3 through 5. Specifically, Chapter 3 presents a methodological tool for operationalizing language exposure in dual language learning contexts. Chapter 4 examines the influence of vocabulary size on speed of auditory word recognition using haptic responses in 16 to 22 month old monolingual and bilingual toddlers. The final set of studies (Chapters 5 and 6) report on monolinguals and bilinguals, respectively, and extend the previous work on word learning to the development of connections between words using gaze responses to assess lexical-semantic networks in the second year of life. Here we investigate how single or dual language exposure and vocabulary size influence the emergence of lexical-semantic priming. Throughout this dissertation, we consistently examine how theoretical models can guide predictions and be revised to account for early development. Together these investigations advance our understanding of bilingual language representation in the semantic domain.