This teacher's guide was developed to help teachers improve learning by incorporating writing into mathematics instruction. The document is divided into three sections. The issue of why writing should be utilized in the mathematics classroom is addressed in the first section. The topics discussed include why students should utilize writing in mathematics class, what additional benefits students obtain from writing in class, how assessment in mathematics utilizes writing, what the National Council of Teachers of Mathematics, business and industry, and colleges and universities say about mathematics and writing. The second section discusses what teachers can do once they decide to incorporate writing in the mathematics class. The topics discussed include steps that can be taken to start using writing in the mathematics class, audiences to whom students can write, purposes for which students write, writing forms that can be employed, methods of evaluating students' writing, how to use students' writing as a diagnostic tool, and methods to improve students' writing. The final section includes examples of writing assignments for mathematics classes and samples of students' responses to the assignments. (MDH)