The main purpose of this study was to research typical language development in children learning two languages simultaneously. One goal of this study was to increase the current knowledge of typical language development in low-income bilingual children under age three. A second goal was to provide the Head Start community with information about bilingual development to ensure cutting edge language services to children raised in bilingual environments. This project involved a collaboration between San Diego State University, University of California (San Diego), and the Neighborhood House Association Head Start Program in San Diego. This two-year research project consisted of three experiments that were conducted with Spanish-English bilingual children at 20 and 28 months of age. The ways in which growth in one language influences growth in the other language at the lexical (vocabulary) and morphosyntactic (grammatical) levels were investigated. The first experiment studied lexical processing using an electrophysiological approach. The second experiment compared vocabulary scores obtained at one time point with grammatical complexity scores obtained eight months later. The third experiment compared grammatical development in the two different languages at the same time point. Eligible children for these studies were contacted through the Southwest Area Head Start and Early Head Start programs run by the Neighborhood House Association.