Little analysis has been made of ethical challenges encountered by health care professionals (HCPs) participating in humanitarian aid work. This is a qualitative study drawing on Grounded Theory analysis of 20 interviews with health care professionals who have provided humanitarian assistance. We collected the stories of ethical challenges reported by expatriate HCPs who participated in humanitarian and development work. Analysis of the stories revealed that ethical challenges emerged from four main sources: (a) resource scarcity and the need to allocate them, (b) historical, political, social and commercial structures, (c) aid agency policies and agendas, and (d) perceived norms around health professionals’ roles and interactions. We discuss each of these sources, illustrating with quotes from the respondents the consequences of the ethical challenges for their personal and professional identities. The ethical challenges described by the respondents are both familiar and distinct for bioethics. The findings demonstrate a need to provide practical ethics support for humanitarian health care workers in the field.