Purpose: This study assesses the factors that contribute to Spanish and English language development in bilingual children. Method: Seven hundred and fifty-seven Hispanic prekindergarten and kindergarten-age children completed screening tests of semantic and morphosyntactic development in Spanish and English. Parents provided information about their occupation and education as well as their children's English and Spanish exposure. Data were analyzed using zero-inflated regression models (comprising a logistic regression component and a negative binomial or Poisson component) to explore factors that contributed to children initiating L1 and L2 performance and factors that contributed to building children's knowledge. Results: Factors that were positively associated with initiating L1 and L2 performance were language input/output, free and reduced lunch, and age. Factors associated with building knowledge included age, parent education, input/output, free and reduced lunch, and school district. Conclusion: Amount of language input is important as children begin to use a language, and amount of language output is important for adding knowledge to their language. Semantic development seemed to be driven more by input while morphosyntax development relied on both input and output. Clinicians who assess bilingual children should examine children's language output in their second language to better understand their levels of performance. (Contains 4 tables.)