Previous research has emphasized both the importance of giving and receiving peer feedback for the purpose of active learning, as well as of university students' engagement in reflection to improve learning outcomes. However, requiring students to explicitly reflect on peer reviewing is an understudied learning activity in higher education that may contribute to the utilization of peer-feedback and promote further learning. In this study, we suggest reflection on peer reviewing as one approach to providing a platform for students to engage in reflective practices and for stimulating active learning in higher education, and to make that learning visible to the educator. We examine 26 undergraduate students' reflections on peer-review to identify categories of reflection and what students have learnt from the peer reviewing process. Our findings reveal six different categories of reflection suggesting students' active engagement in learning and pointing to the ways educators can direct and instruct students how to reflect. We discuss how these findings can inform university lecturers in the use of reflection upon peer reviewing as a pedagogical tool in higher education.