This study examined the efficacy of the TELL preschool curriculum for promoting the acquisition of oral language and early literacy skills in preschool children from low-income families. National assessment results indicate a sustained and persistent achievement gap in the reading skills of children from more- versus less-advantaged backgrounds (National Assessment of Educational Progress, 2019). In fact, over the past 20 years gaps between poor White students and their poor Black and Hispanic peers grew (Paschall, Gershoff, & Kuhfeld, 2018). Historically, the percentage of children in a school who live in poverty is the single best predictor of standardized reading comprehension scores at all grade levels (Ransdell, 2011). As a group, children from economically disadvantaged households start school with lower language skills (Dickinson, 2011; Hanson et al., 2011), poorer phonological sensitivity (Bowey, 1995; Raz & Bryant, 1990), and poorer print concepts than their more-advantaged peers (Smith & Dixon, 1995). Provision of high-quality preschool programs for children from families living in poverty is a national priority, predicated on a growing body of evidence demonstrating that preschool can have positive, long-lasting benefits for all children. Further, children from economically disadvantaged families may benefit more from high quality preschool programs than children from higher SES families (Schweinhart, 2013). The "Teaching Early Literacy and Language" (TELL) Tier 1 curriculum package was designed to promote high quality preschool programs. Research indicates that high-quality programs include (a) responsive teaching and classroom environments that offer a variety of learning opportunities (Mashburn & Pianta, 2010; Pianta, La Paro, & Hamre, 2008), (b) implementation of empirically-based curricula with sufficient "active ingredients" to bring about positive impacts on children's language, literacy, social, and cognitive skills (Diamond et al., 2013), (c) teaching that matches children's developmental levels, and (d) provision of effective professional development that is aligned to desired child outcomes (Pianta, et al., 2009). The TELL package includes each of these important components. Method: Preschools serving low-income children in the Phoenix metropolitan area were recruited for the study. Most were Head Start or Title 1 programs. We implemented an RCT with a three-cohort design across three years, with preschool teachers serving as the unit of assignment. Teachers were stratified by school/agency and then randomized to the experimental (TELL) or business-as-usual (BAU) condition. Enrollment for the three cohorts was 500 children (285 TELL and 215 BAU) from 63 preschool classrooms (34 TELL and 29 BAU). Because maternal education and children's academic outcomes, cognitive development, and speech and language skills are often significantly correlated (e.g., Magnuson, Sexton, Davis-Kean, & Aletha, 2009), all analyses included maternal education as a covariate. Child outcomes were measured using (a) Curriculum-Based Measures (CBMs) administered at six unequally spaced time points across the school year coinciding with the end of instructional unit; (b) the Preschool Early Literacy Indicators (PELI; Kaminski, Abbott, Bravo, Latimer, & Good, 2014) that measured children's alphabet knowledge, vocabulary, comprehension, and phonological awareness at three time points during the year (beginning of year, January, end of year), and (c) the Listening Comprehension and Phonological Coding subtests of the Woodcock-Johnson IV Test of Achievement (Schrank, McGrew, Mather, & Woodcock, 2014) administered at the beginning and end of the year. Results: We tested for but found no cohort effects for Years 1, 2, and 3; thus, cohorts were combined into a single sample for the analyses. Because of COVID-19, we were not able to collect end of year data for Cohort 3. The missing data due to COVID-19, as well as any missing data because of child absences, was treated as missing at random. For the CBMs and the PELI, linear mixed models using a normal distribution for the response variable was employed with fixed effects factors for treatment and time, random effects for school, and random coefficients at the child level. Child was crossed with time, but children were nested within classrooms, and classrooms were nested within treatment level. A treatment by quadratic time trend interaction was investigated for each CBM, but none were significant. The mixed model specified random coefficients for intercept and linear slope, so each child has his or her own growth trajectory, and the random intercept and slope terms also accommodate heterogeneous variance and covariance across time. As shown in Table 1, results indicated a significant time x condition effect for the Letter Sounds, Print Awareness, and Beginning Sounds CBMs, with growth in TELL classrooms significantly higher than growth in BAU classrooms. The effect for the Narrative Story Retell CBM was not significant. As shown in Table 2, results indicated a significant time x condition effect for the PELI Alphabet Knowledge and Phonological Awareness subtests in favor of TELL classrooms, but no significant effect for the Vocabulary or Comprehension subtests. The mixed model for the Woodcock-Johnson IV subtests included a random effect for classroom and a fixed effect for occasion instead of random intercept and slope terms for trend over time. As shown in Table 3, results indicated a significant between-group difference in favor of TELL classrooms on the Woodcock-Johnson IV Phonological Coding subtest but no differences on the Listening Comprehension subtest. Discussion: The use of an efficacious Tier 1 language and literacy curriculum has the potential to increase the quality of early childhood programs and to have positive, long-lasting effects on children's development and success in elementary school. The present study demonstrated TELL's effectiveness for supporting learning of essential oral language and early literacy skills including print and phonological awareness (CBMs), alphabet knowledge and phonological awareness (PELI), and a distal, norm-referenced measure of phonological coding (WJ-IV). The findings are consistent with those from two previous TELL RCTs with children with developmental speech and/or language impairments (Wilcox, Gray, Guimond, & Lafferty, 2011; Wilcox, Gray, & Reiser, 2013, Gray, Wilcox, & Reiser, 2015). Taken together, evidence shows that the TELL Tier 1 curriculum is efficacious for teaching oral language and early literacy skills to preschoolers from low-income households.