Health professionals are involved in humanitarian assistance and development work in many regions of the world. They participate in primary health care, immunization campaigns, clinic- and hospital-based care, rehabilitation and feeding programs. In the course of this work, clinicians are frequently exposed to complex ethical issues. This paper examines how health workers experience ethics in the course of humanitarian assistance and development work. A qualitative study was conducted to consider this question. Five core themes emerged from the data, including: tension between respecting local customs and imposing values; obstacles to providing adequate care; differing understandings of health and illness; questions of identity for health workers; and issues of trust and distrust. Recommendations are made for organizational strategies that could help aid agencies support and equip their staff as they respond to ethical issues.