Communication has long been emphasized in standards-based instruction (NCTM 1991; 2000), yet little distinction has been made between oral and written forms. Nonetheless, both the mathematics and the English language arts Common Core State Standards (CCSS) documents continue to hint at the importance of writing mathematically (CCSSI 2010). The third of the Standards for Mathematical Practice (SMP 3) (CCSSI 2010a) implies that students are expected to write because they are to read others' arguments. The Writing standards (CCSSI 2010b) also call for elementary school students to write across the curriculum, which includes mathematics. Specifically, the English language arts (ELA) standards (CCSSI 2010c) list three writing types. Although narrative writing may be more central to other subjects, the remaining two apply to mathematics. In this article the author offers instructional recommendations based on her experience with mathematical writing in curriculum-development work and guiding writing endeavors.