In the last decade, researchers have focused more on how to provide instructional supports for mathematics writing (Powell et al., 2017). In this synthesis, we examined 22 studies about mathematics writing to determine the overall mathematics-writing and mathematics outcomes of mathematics-writing instruction, the mathematics-writing and mathematics outcomes based on study and instructional features, the methods used within mathematics-writing instruction, and the methods used within studies involving students with mathematics difficulty (MD). We conducted a comprehensive review of peer-reviewed studies, published since 2000, focused on instruction in mathematics writing in Kindergarten through Grade 12. We included studies with and without students with MD. Results demonstrated positive student outcomes for mathematics-writing instruction. Furthermore, instructional methods for mathematics writing frequently aligned with practices used for mathematics instruction and writing instruction. Although we identified positive outcomes related to mathematics-writing instruction, and we would recommend for practitioners to provide instruction in mathematics writing, results should be interpreted with caution. Additional high-quality research on mathematics-writing instruction is needed to verify and extend on the results from this synthesis.