Children's early understanding of mathematics provides a foundation for later success in school. Identifying ways to enhance mathematical instruction is crucial to understanding the ideal ways to promote academic success. Previous work has identified mathematical language (i.e., the words and concepts related to early mathematical development such as more, same, or similar) as a key mechanism that can be targeted to improve children's development of early numeracy skills (e.g., counting, cardinality, and addition). Current recommendations suggest a combination of numeracy instruction and quantitative language instruction to promote numeracy skills. However, there is limited direct support of this recommendation. The goal of the proposed study is to compare the unique and combined effects of each type of instruction on children's numeracy skills in the context of picture book reading. We randomly assigned 234 children (ages 3-5) to one of four conditions where they worked with trained project staff who read picture books targeting: (a) quantitative language only (e.g., more or less), (b) numeracy only (e.g., cardinality, addition), (c) combined quantitative language + numeracy, or (d) nonnumerical (active control) picture books. Results revealed no significant effects of the quantitative language only or numeracy only conditions, but mixed effects of the combined condition. These findings indicate that more work is needed on how mathematical language and numeracy instruction should best be delivered to preschool children. Educational Impact and Implications StatementSignificant research has highlighted the importance of quantitative language in children's early mathematics development. The findings from this study raise questions as to how to best implement such interventions. Namely, in contrast to prior work, the current interventions targeting one of the two domains did not exhibit positive results. However, mixed effects were found for the condition that combined both domains. Future work on how to best structure combined instruction is needed.