There is growing acceptance of the concept of health security. However, there are various and incompatible definitions, incomplete elaboration of the concept of health security in public health operational terms, and insufficient reconciliation of the health security concept with community-based primary health care. More important, there are major differences in understanding and use of the concept in different settings. Policymakers in industrialized countries emphasize protection of their populations especially against external threats, for example terrorism and pandemics; while health workers and policymakers in developing countries and within the United Nations system understand the term in a broader public health context. Indeed, the concept is used inconsistently within the UN agencies themselves, for example the World Health Organization's restrictive use of the term 'global health security'. Divergent understandings of 'health security' by WHO's member states, coupled with fears of hidden national security agendas, are leading to a breakdown of mechanisms for global cooperation such as the International Health Regulations. Some developing countries are beginning to doubt that internationally shared health surveillance data is used in their best interests. Resolution of these incompatible understandings is a global priority.