The role of language in mathematics teaching and learning is increasingly highlighted by standards and reform movements in the US. However, little is known about teachers', and especially early career teachers' (ECTs) practices and understandings related to language in mathematics instruction. This multiple case study explored the language-related understandings and practices of six ECTs in diverse elementary classrooms. Using iterative cycles of analysis, we found that all ECTs regularly attended to students' mathematical vocabulary use and development. Yet, there was variability in ECTs' focus on how to teach mathematical vocabulary, expectations for students' precise use of mathematical terminology, and the use of multiple languages during instruction. These findings indicate that ECTs need more targeted support during teacher preparation and early career teaching in order to better support all students' language development in the mathematics classroom. Rationale In recent years, a range of mathematics education reforms and standards initiatives in the United States and internationally have drawn increasing attention to the role of language in teaching and learning mathematics (National Council of Teachers of Mathematics (NCTM), 2010, 2014; National Governor's Association Center for Best Practices & Council of Chief School Officers (NGACBP & CCSO), 2010; Adler, 2002; Parveva et al., 2011). Students are expected to use language in various ways in mathematics classrooms; for example, unpacking problem statements that include complex phrases, explaining and justifying thinking, and discussing connections between multiple representations (e.g., objects, drawings, diagrams) (Moschkovich, 2012; NGACBP & CCSSO, 2010). In the U.S., the Common Core Standards for School Mathematics (CCSSM) have increased language expectations for all students with the emphasis on explaining reasoning over performing procedures. For example, Mathematical Practice 3, Construct viable arguments and critique others' reasoning, requires students to communicate mathematical justifications, and respond to others' ideas. Mathematical Practice 6, Attend to precision, requires that students "communicate precisely to others,…use clear definitions in discussions with others and in their own reasoning" (NGACBP & CCSSO, 2010, p.7). These standards align with current research on language and mathematics that has found that children's language skills have a complex and intertwined relationship with their developing mathematical skills (Bernardo, 1999; Clarkson, 2007; Martinello & Wolf, 2012; Purpura & Ganley, 2014), and thus are relevant beyond the U.S. This emphasis on language has also expanded expectations for teachers, as they must attend to students' mathematical communication skills as well as conceptual understanding (Gutierrez, 2002; Santos, Darling-Hammond, Cheuk, 2012). This can be challenging, particularly in linguistically diverse classrooms where students may receive instruction in a language other than their home languages (Moschkovich, ...