Four studies explored patterns of language growth and processing in 64 19-31-month-old bilingual children acquiring English and Spanish. In the first study, cross-sectional and longitudinal methods revealed significant relationships between vocabulary size and grammatical development, replicating previous studies with monolingual children. The compositions of children's lexicons in each language were linked to vocabulary size in that language. With few exceptions, utterance length and complexity and the emergence of closed class and predicate terms in each language were linked more closely to vocabulary size in the same language than to total conceptual vocabulary (TCV) size. In the second study, performance on English and Spanish sentence repetition tests was compared for bilingual and monolingual children matched for vocabulary size in each language. The performance of the bilingual children was similar to that of their monolingual controls, indicating links between grammatical ability and vocabulary development within the same language. The slightly better performance of the bilingual children on a few comparisons suggested some degree of cross-linguistic bootstrapping. In the third study, event-related potentials (ERPs) were used to investigate the neural activity of bilingual children as they processed known and unknown words. There were differences in the timing and distribution of the ERP amplitude differences to known vs. unknown words for each language, and differences in these effects when children with larger TCV sizes were compared to those with smaller TCV sizes. These results thus replicated studies with monolingual children that found links between vocabulary development and the organization of ERP effects, although the exact patterns were different for the bilingual and monolingual children. In the fourth study the effects of mixed vs. blocked language testing conditions on ERP patterns were investigated. Results indicated that some of the differences in ERP effects between the monolingual and bilingual children noted in the third study may have been due to processing demands created by the mixed-language testing condition in which the bilingual children were tested. The results of these four studies support experience-based accounts of early language acquisition, and further establish the use of combined behavioral-neural imaging approaches for studying language development in bilingual toddlers. (author abstract)