In this chapter, the authors situate mathematical writing within the context of discourse, define mathematical writing, describe the classroom environment that fosters mathematical writing, and share experiences engaging elementary students in mathematical writing. At their core, the types of mathematical writing are defined by the purposes for which students write in the mathematics classroom. The authors further describe the types of mathematical writing teachers prompt and are nurtured by the classroom environment. The insights shared draw from the authors' previous and current research endeavors, advancements in curriculum development, collaborations with mathematics coaches, and experiences teaching elementary students and supporting preservice and inservice teachers. They end the chapter by highlighting some directions for future research to expand our understanding of this crucial area in mathematics education.