Writing is the ability to compose text effectively for different purposes and audiences. When many of us reflect on our own school experiences, we recall writing in English and history classes, but not in mathematics. Math classes previously relied on skill-building and conceptual understanding activities. Today, teachers are realizing that writing during a math lesson is more than just a way to document information; it is a way to deepen student learning and a tool for helping students gain new perspectives. They realize, too, that students whose strengths are language-based--and many are--use writing as the key to understanding other disciplines, especially mathematics. Like most things, learning to write well requires instruction and practice. In this booklet, the author aims to nudge secondary math teachers who are thinking about using writing in their classrooms more extensively and to encourage those who want to begin. Section One gives a brief background that answers the question you may be wondering: Why write in mathematics? Section Two describes the existing role of writing in the mathematics curriculum, and Section Three provides strategies and ideas to put into practice right away. (Contains 1 footnote.)