This study seeks to illuminate and describe the experience of students engaging in mathematical portfolios. In particular, this study reveals key insights into how students demonstrate their mathematical identity and metacognition. Portfolio assessment has been described as an intentional collection of evidence that captures student "accomplishments, efforts, and progress" (p. 487) over time (Gillespie et al., 1996). Portfolios for writing have a sporadic history of use in schools (Elbow & Belanoff, 1997). However, research on the use of portfolios as direct assessment artifacts is at best tangential in mathematics (e.g., Chang et al., 2011; Fritz, 2001). Student mathematics portfolios provide natural opportunities for metacognitive experiences and recognition of self-improvement in mathematics (Wiggins, 1990).This study takes a qualitative phenomenological approach, using teacher interviews, student mathematical work samples, and student reflective responses to describe the evolution in student identity and metacognition through the implementation of student mathematical portfolios in a teacher's middle school class. The research partnership included providing teachers professional development toward implementation of student mathematical portfolios in classroom use followed by data collection and analysis. My research questions are as follows: 1. How does the experience of using a portfolio system in mathematics class contribute to students' mathematical identity? 2. How does the experience of using a portfolio system in mathematics class contribute to students' metacognition? [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://bibliotheek.ehb.be:2222/en-US/products/dissertations/individuals.shtml.]