Context: The psychological realm of medical students' moral experiences is explored tangentially in medical education literature, often in the context of ethics or professionalism education. This study deepens our understanding by (a) investigating the nature of moral dilemmas experienced at the onset of clinical practice, (b) exploring students' emotional response to these dilemmas, and (c) examining how students perceive the influence of these dilemmas on their professional development. Methods: This is a cross-sectional qualitative study carried out in 2017 that applied thematic template analysis to individual interviews performed with last-year medical students. The interviews followed the drawing of a Rich Picture representing moral dilemmas experienced by medical students at the onset of clinical practice. Results: Moral dilemmas have four intertwined dimensions. The first relates to students' struggle to prioritise, balance and apply conflicting moral values; the second comprises the clash between students' inner motivation and the external constraints that limit the moral action; the third refers to the conflict between students' current attitudes with the desired/idealised attitudes of the doctor they intend to become; and the fourth corresponds to weighting conflicting ethical principles during the moral decision. Students' emotional responses are intense and long-lasting, and with a remarkable residue effect, particularly when the moral decision does not align with their moral beliefs. Moral dilemmas are impactful experiences that affect the professional development of medical students and can culminate in both detachment and growth in moral courage. Conclusion: Moral dilemmas are memorable, complex and emotionally intense experiences that impact the professional development of medical students. Understanding students' moral dilemmas can help educators to devise pedagogical activities to anticipate and reflect on these experiences. These activities should happen under the guidance of a non-judgemental facilitator, capable of listening and legitimating students' thoughts and feelings while providing insights to nurture their professional development.