Literacy instruction in science and math in elementary education lays a critical foundation for later content literacy development and the learning of content subjects in middle and high school. However, limited research has investigated the theoretical basis of instructional practices for science and math literacy instruction at the elementary level. Extending Wright et al., (2016), the present systematic review aims to critically examine the connection between theory and practice for K-5 science and math literacy instruction. Instructional practices recommended in the "Reading Teacher," a flagship practitioner-oriented journal published by the leading literacy education association, "International Literacy Association," were analyzed to identify the underlying theories. Our findings revealed that most of the science and math literacy practices recommended to elementary reading teachers are well aligned with social theories, which dominate the field of literacy education and conceptualize literacy as a social practice. Following social theories of literacy development, students are encouraged to make connections between the text and their personal interest and life experiences. The current study also revealed gaps between theory and practice in elementary science and math literacy instruction, where key theories were not or underrepresented in practices. Further strengthening the connection between theory and practice requires a joint effort between researchers and practitioners.