Traditionally, teachers assess students' physics understanding through lab activities, responses to open-ended word problems, and tests. But there is another way to measure student understanding, one in which students apply their learning to the world around them. This article shows how to implement student portfolios, which allow students to set goals they can monitor throughout the year and actively participate in assessment. When students build portfolios, they can evaluate and reflect on their own work, promoting engagement with the course and content, and teachers can better assess students' goal movement and see growth in students' conceptual understanding.