The purpose of this qualitative study is to explore students' self-reported growth of mathematical understanding resulting from writing about the historical development of a mathematical concept. Using excerpts from undergraduate students' written reflections about their experiences with the task of writing about mathematics, this study describes three emergent themes concerning how these students explicitly assessed their own learning: self-awareness of initial understanding, learning from the task of writing, and deeper understanding of mathematics. The results indicate that writing about the historical development of mathematics encouraged students to acknowledge their existing mathematical understanding as well as extend or develop new understandings. Furthermore, writing about mathematics can serve as a viable assessment of individual student learning and provide insight into what students understand independent of formal assessment based on procedural skills.