Is English-only or dual-language instruction in preschool more effective at preparing dual-language learners (DLLs) for kindergarten and elementary school? To date, just a handful of studies have explored this critical question (Barnett, Yarosz, Thomas, Jung & Blanco, 2007; Durán, Roseth & Hoffman, 2010; Farver, Lonigan & Eppe, 2009). These studies have been important to educators as they suggest that dual-language preschool programming may be one method of boosting the school readiness of DLLs, many of whom lag behind their native English speaking peers in math and reading upon arriving to kindergarten (Reardon & Galindo, 2009). In these studies DLLs in dual-language preschools have made similar English language improvements, while making greater Spanish language improvements, compared to their counterparts in English-only preschools. While these studies indicate possible benefits of dual-language instruction, they are by no means definitive, largely because the results may be due to factors other than simply the mode of instruction and the study sample sizes have been relatively small. The goal of the Language Instruction Research Project is to study the immediate and longitudinal effectiveness of two methods of language delivery – dual-language and monolingual English immersion instruction. It uses a randomized control trial (RCT) design to estimate the impacts of a dual-language approach (Spanish and English) on children’s language and cognitive development in both languages. In addition this study examines whether classroom characteristics moderate the association between language of instruction and child outcomes.