Recent reforms in science education have supported the inclusion of engineering and their practices in K-12 curricula. To this end, many classrooms have incorporated engineering units that include design challenges. Design is an integral part of engineering and can help students think in creative and interdisciplinary ways. In this study, we examined students' conceptions of design during and after participation in a design-based science curriculum unit. Our study was guided by the following research question: What are students' views of design after participation in an engineering design-based science curriculum unit and how are these views reflected in their enactment throughout the unit? Using a qualitative approach, we examined students' conversations throughout the enactment of the curriculum and interviews conducted after the completion of the unit. We found that students had complex and diverse views of design, and these views were reflected in their group discussions throughout the curriculum and design challenge. Students most frequently expressed design as learning and as a process of integration into a coherent whole. These aspects of design were also frequently observed in students' conversations during the unit. Interestingly, we found evidence of students demonstrating several aspects of design throughout the curriculum that were not explicitly expressed during the student interviews. Taken together, these findings support the complex nature of design as seen at the middle school level.