Assessment of students' critical thinking and problem solving in engineering is a real challenge for classroom teachers and researchers. Yet, students demonstrate evidence of learning through multiple means, including written reflections. The purpose of this study was to explore how students in grades 5 and 7 reflect on what they had learned about engineering design practices in comparison to their previous understandings. The researchers applied qualitative content analysis to analyze student responses to engineering notebook prompts that asked students to reflect on their understanding of the problem and how to design solutions. Data were collected from two classrooms (n = 47) that had implemented integrated STEM curricula. The results of this study indicate that students were able to reflect meaningfully on their engineering practices and how their understanding of what it meant to design had changed. The notebooks provided an opportunity for students to demonstrate evidence of their learning through reflection on their own design practices. The findings suggest that teachers and curriculum developers can use reflection as a means to help students connect their own learning to informed design practices, which may help students move toward being independent informed designers. Future research should consider how teachers can use notebooks to provide feedback on engineering practices.