Teachers' beliefs influence their teaching practices. Given the U.S. Secretary of Education's push to increase multilingualism, this systematic literature review examines teachers' beliefs about language diversity and multilingual learners in relation to teacher experiences, teaching practices, and external factors. Using 17 search terms, we searched seven databases for empirical, peer-reviewed studies that examined K-12 teachers' language beliefs. We analyzed 63 studies that met our criteria using a framework of teacher beliefs. Our findings indicate that researchers use myriad terms to capture the participating teachers' beliefs. These terms could be grouped into dominant language, subtractive, additive, and pluralist beliefs. We also found connections between teachers' beliefs on one side and their life experiences, instructional practices, and external factors, such as high-stakes tests and school contexts, on the other side. We added teachers' beliefs about families, content and knowledge, and diversity to the framework, as well as external factors.