Background: To succeed in school, young children must acquire the domain and vocabulary knowledge needed to read complex texts. Despite the growing literacy demands in the 21 century, fewer than 5% of US school children in the early elementary grades can evaluate complex nonfiction texts and there are larger gaps based on family income and student ethnicity on text comprehension outcomes rather than basic word reading skills (Reardon, Valentino, & Shores, 2012). These early difficulties in children's ability to read for understanding have consequences in the later elementary school grades. For example, the 2019 administration of the National Assessment of Educational Progress (U. S. Department of Education, 2019) revealed that the reading comprehension performance of fourth-graders has declined over time, particularly for low-income children and struggling readers. Why are so many elementary grade children struggling to read complex informational texts with understanding? The limited opportunities for children to participate in comprehensive and sustained content literacy interventions may play a pivotal role (Pearson, Palinscar, Biancarosa, & Berman, 2020). Researchers have long argued that more comprehensive intervention approaches are needed to help children build, organize, and use rich inter-connected networks of vocabulary knowledge to read complex text (Hirsch, 2016). For example, domain-specific vocabulary is hierarchically organized where mastery of early concepts lays the foundation for future learning, multi-year intervention studies are critical to helping children grow vocabulary networks through multiple exposures in varying contexts that reveal and deepen word meanings (McKeown et al., 2017; Perfetti, 2007). Indeed, the broader rationale for this multi-year interventions is rooted in the hypothesis that knowledge begets more knowledge and high-quality instruction Tier I instruction in second-grade content classes can build upon and extend what students were taught in first-grade. Purpose/Objective Research Questions: The purpose of this mixed methods study was to examine whether a multi-year implementation of a content literacy intervention (MORE) could promote transfer effects on second-graders' science reading comprehension outcomes. Treatment group children participated content literacy instruction that provided a framework for learning more general schemas in grade 1 science (animal survival) that were foundational to learning more specialized topics in grade 2 science (mass extinctions of dinosaurs). The MORE Grade 2 lessons focused on a single theme and organizing questions: How do paleontologists study dinosaurs? The unit was designed to be taught over a 5-week lesson sequence. Treatment teachers received a pacing calendar to reference the instructional sequence and were given substantial flexibility in the pacing of instruction to teach the 5-week lesson sequence and activities from mid-October to early February. Teachers interleaved 9 activities during a single lesson sequence MORE, as shown in Figure 2. Each lesson included into two sections. The first section focused on building domain knowledge through interactive read-alouds and concept mapping. Interactive read-alouds involved reading thematically related informational texts (20 minutes), a structured equitable academic discussion and/or concept mapping activity (15 minutes), and mapping their progress and goal mastery status in each unit (5 minutes). The second section focused on transferring vocabulary knowledge. Students used an inquiry based approach to build morphological awareness of words in the concept mapping. Students engaged in argumentation and collaborative research using text features (e.g., compare and contrast) to obtain additional information on given concepts during reading and writing activities (20 minutes) and participated in in-depth academic group discussions while incorporating relevant details and evidence from their research (20 minutes). We addressed the following research questions: (1) To what extent does a two-year implementation of the MORE content literacy intervention improve second graders' (a) science vocabulary knowledge depth and (b) science content comprehension? Goal #1: Longitudinal studies (2) To what extent does the magnitude of the treatment effect differ across the near, mid, and far transfer measures of science content comprehension? Goal #2: Transfer effects on reading comprehension; and (3) To what extent do initial reading comprehension ability moderate effects on science vocabulary knowledge depth and science content comprehension outcomes? Goal #3: Matthew effects, like previous MORE studies. Population/Participants/Settings: Table 1 displays descriptive characteristics of participating students. Black and Hispanic students comprised the majority of the student sample. Students were from socioeconomically diverse neighborhoods and nearly 20% were designed as having limited English proficiently. Importantly, there were no statistically significant baseline differences between the treatment and control group on measured teacher and student characteristics at the individual level and the school level. Research Design: Thirty elementary schools in one urban school district located in the southeastern United States participated in this two-year longitudinal study. Prior to program implementation, we pre-registered the study design, conducted a power analysis to determine the minimum detectable effect size, and implemented a blocked, school-level randomized controlled trial (Authors, 2020). In particular, schools were blocked (i.e., stratified) by demographic and achievement characteristics and then randomized to conditions for two consecutive years. Assuming an alpha level of 0.05 (two-tailed tests) on all impact models, a pretest reading covariate (R[superscript 2] = 0.50), and 80% power, the minimum detectable effect sizes were 0.25 across the primary student outcomes. Data Collection/Analysis/Results: At the end of second-grade, students were administered a 2-item semantic association task to assess vocabulary depth and a passage comprehension test that included concepts that were taught during the second-grade lessons. Hierarchical linear models revealed a statistically significant impact on transfer measures of science content reading comprehension (ES = 0.18). Comparison of effects across passages revealed that there were larger effects on measures of near transfer (ES = 0.23) and mid-transfer (ES = 0.16) than on far transfer (ES = 0.05). There were no statistically significant effects on measures of vocabulary knowledge depth (ES = 0.06). Fidelity analyses based on surveys indicated that treatment group teachers were more likely to support incidental word learning by providing opportunities for children to hear and see words through interactive read-alouds and discussions involving science words (ES = 0.73). Conclusion: Both direct instruction of semantically related science words and incidental exposure through shared reading and discussions may support students as they use their vocabulary knowledge to read complex science texts with understanding.