The present dissertation had three main goals: 1) To examine similarities and differences in monolingual and bilingual vocabulary acquisition during a critical period of development 2) To examine how processing speed and language exposure differentially impact vocabulary development and the acquisition of translation equivalents (TEs; words in each language that mean the same thing, such as dog in English and chien in French) in bilingual children during the second year of life, and 3) To acquire a more comprehensive understanding of the stability and continuity of early bilingual vocabulary development by using a direct measure of vocabulary comprehension and processing speed (the Computerized Comprehension Task; CCT) in conjunction with parent reported vocabulary (the MacArthur Bates Communicative Development Inventory; CDI) longitudinally. In order to address these goals, data was collected on two samples of children, one monolingual and one bilingual, at three different developmental time points. Three manuscripts were then written based on this data, and are included as part of this dissertation. 
The first manuscript, published in 2016 in the Journal of Child Language, examines the receptive vocabulary development of a sample of French-English bilingual and French monolingual children at 16 months of age. This manuscript not only compares the bilingual sample’s receptive vocabulary development and word processing speed to that of their monolingual peers using data from the CCT and CDI, but it also examines the emergence of TE acquisition, and investigates within- and cross-language relations between vocabulary size and reaction time (RT). The findings from this paper suggest that bilingual receptive vocabulary development is largely on par with that of monolingual development, and that learning more than one language from birth does not hinder children’s speed of lexical access. Importantly, it also emphasizes the link between language exposure, vocabulary size, and processing speed, and shows that children with larger vocabularies tend to be faster at processing words.
The second manuscript included in this dissertation was published in Bilingualism: Language and Cognition in 2016. It follows the same samples of children from 16 to 22 months of age, and focuses on growth in vocabulary comprehension and production, as well as changes in language exposure and processing speed across waves. Importantly, this study is the first to investigate vocabulary growth in a sample of French–English bilingual toddlers using a longitudinal design in conjunction with a direct measure of vocabulary development. Although both receptive and expressive vocabulary development began slowly in the bilingual sample with learning divided across languages, over time these toddlers acquired approximately as many new words as their monolingual peers in their dominant language, and exhibited a total vocabulary size that was equivalent to, or larger than, their monolingual peers. Furthermore, children’s processing speed increased across waves, and RT on the CCT at 16 months emerged as a significant predictor of receptive vocabulary size at 22 months for the bilinguals. Importantly, both within- and cross-language relations emerged between language exposure, vocabulary size, and processing speed for the bilinguals, once again emphasizing the complex interplay between these variables early on in development. 
Finally, the third manuscript included in the present dissertation was published in the Journal of Child Language in 2017. It focuses on productive vocabulary development and the acquisition of TEs in our French-English bilingual sample across three developmental time points, at 16, 22, and 30 months. It also compares a direct measure of TE development with parent report in a separate sample of 24-month-old French-English bilinguals. This is the first study to longitudinally investigate the impact that changes in language exposure and vocabulary size have on TE development during the second and third years of life. It is also the first study to compare a direct measure of TE comprehension with parent report during the second year of life. This manuscript shows that the acquisition of TEs is a gradual process that begins early on in bilingual development. It also provides evidence for the relation between quantity of language exposure and TE development, but shows that the ratio of L1 (dominant) to L2 (non-dominant) vocabulary is a better predictor of TE development than L2 exposure alone in young bilinguals. Lastly, this manuscript emphasizes the importance of using both direct and indirect measures of early vocabulary comprehension and TE development, as it shows that parents of bilingual children may have a tendency to over-report their child’s receptive word knowledge when completing vocabulary checklists.