Writing in mathematics can improve procedural knowledge and communication skills and may also help students better understand and then remember problems. The majority of mathematics teachers know that they ought to include some writing assignments in their instructional plans, but the challenge of covering the curriculum and the time required to plan and institute a learning activity of this nature are high hurdles. Researchers found that writing activities are most effective when they use an explicit and well-designed model and when they work in teams of teachers to support one another. The purpose of this article is to show how teachers have implemented a portfolio assessment model at an urban charter school with impressive results. It also describes some of the decisions that teachers have to make to use a portfolio assignment in class. The authors offer an alternative model for a portfolio assignment and show how they can teach students to create portfolios successfully at Prospect Hill Academy Charter School in Cambridge, Massachusetts. (Contains 5 figures.)