Background: Little is known about how students engage in ethical decision-making, especially when designing in messy, real-life contexts. To prepare ethically competent engineers, educators need a richer understanding of students' ethical decision-making throughout the course of the design process. Purpose/Hypothesis: This study examines students' intuitive ethical decision-making as it emerges throughout the design process as well as when and how students engage in ethical reflection. Outlining these processes enables educators to better structure and support students' ethical reasoning. Design/Method: We conducted 103 semi-structured interviews with students in a multidisciplinary service-learning program. To capture how ethical decision-making unfolded over time, we sampled 13 students who had participated for multiple semesters on the same projects. The resulting 30 interviews were transcribed, coded, and thematically analyzed. We then explicated when and how students appeared to grapple with the ethical principles of beneficence, nonmaleficence, autonomy, and justice. Results: The findings trace which ethical principles emerged as salient in each phase of the design process as well as what conditions and activities stimulated students' reflection on their ethical decision-making. Conclusions: Although certain phases of the design process appear to prompt consideration of specific principles, students' interactions with users and project partners appeared to stimulate the most reflection on their ethical decision-making. We discuss how educators can leverage these and other reflection triggers to better structure and support students' ethical reasoning as well as strategies for making intuitive processes more explicit.