Health personnel volunteering for humanitarian assistance assignments work in increasingly dangerous situations and increasingly complex roles. A qualitative analysis of interviews with returning Swedish aid workers, who collectively had been on 74 missions in 32 different countries, revealed that they felt positive about their contribution, but experienced high levels of stress and frustration. They were also surprised and inadequately prepared for tasks that fell outside their professional health care training, including ones demanding pedagogic and management skills. The volunteers perceived their success on humanitarian assistance assignments as being affected not only by their own professional competence and special preparatory training, but also by many other factors. In particular, recruiting organizations could improve volunteer performance by accepting only experienced professionals, requiring special preparatory training, clarifying the exact nature of the work, and providing better support during the assignment. Further analysis of humanitarian assistance as a complex and dynamic system involving multiple 'actors' could lead to improved understanding and better performance.