Introduction Public health communication is the production and exchange of information to inform, influence or motivate the target audience to enable desirable, sustainable health practices at individual, community and institutional levels. This paper explores key issues relating to health communication in refugees and other displaced populations, i.e. the hardly reached. (1) Though there are several challenges there are also many opportunities and lessons learned that can be applied to existing programmes. In 2007, the number of people of concern to the Office of the United Nation's High Commissioner for Refugees (UNHCR) included 9.9 million refugees and 12.8 million internally displaced persons. A refugee is a person who is outside his/her country of origin owing to a well-founded fear of persecution and is unable to avail himself/herself of the protection of that country. Internally displaced persons often leave their homes for the same reasons as refugees, including avoiding armed conflict or generalized violence but also natural or human-made disasters; unlike refugees, internally displaced persons have not crossed an internationally recognized national border. Displacement is often assumed to be short-term but refugees can remain within host countries for many years, during which there is a need for effective health communication. In any well-planned communication intervention the approach must be appropriate to the target audience. While this applies to all populations, challenges and opportunities in health communication targeting displaced populations vary depending on, but not limited to, the stage of displacement, physical environment and location (urban, rural and camp-based settings), the sociocultural context and the degree of diversity within the displaced population. Phases of displacement Health communication is a necessary--but often overlooked--aspect of the emergency response. The aim is to ensure that the most vulnerable have access to essential and accurate information about key practices, available services and relief supplies to prevent the main causes of morbidity and mortality and to prevent abuse and exploitation. (2) Low cost, low-technology communication systems are often the most practical and effective during the initial phases. Megaphones, battery-operated public address systems, billboards and community radio can quickly and widely disseminate messages as well as provide opportunities to promote participation. Large gatherings for distribution of relief items provide further opportunities for information dissemination. As the situation stabilizes, efforts should be expanded to promote sustained behaviour change and create a supportive environment. Areas of focus will depend on the local epidemiology and pre-existing knowledge and practices. Messages, activities and materials should be planned, implemented and monitored with the affected communities. Participation in all phases allows community input in decision-making, promotes ownership and community capacity and helps affected communities achieve a sense of normality. Participation begins with formative assessment or audience research conducted before the start of an intervention to gauge the needs, challenges and opportunities within the target population to guide programme activities; influential community members, leaders, women's and youth representatives are essential partners in this process. During the repatriation phase, health communication must be targeted towards the most immediate needs, including information on availability and location of essential services on return. Communication should begin pre-departure; messages should be short and focused as periods of contact with returning refugees are limited and returnees are often preoccupied with other issues. Reception centres in Afghanistan promote landmine awareness to returnees through well-designed and clearly marked visual displays and a film. …