Since September 11, 2001, and the anthrax attacks that followed, the federal government has invested over $6 billion in efforts to increase the United States' ability to prepare for and respond to public health emergencies. However, it is unclear whether these efforts have improved the nation's ability to respond to a bioterrorist attack, influenza pandemic, or other large-scale public health emergency. The problem lies, in part, in the lack of agreement about what public health emergency preparedness (PHEP) is and how it should be measured. To help remedy this, the Department of Health and Human Services asked the RAND Corporation to convene an expert panel, which convened in February 2007, to develop a clear and widely applicable definition of PHEP that can provide common terms for discussion and establish a basis on which to develop a small core of critical standards and measures. This paper provides the candidate definition of PHEP developed by the panel, describes its key elements, and discusses the approach that the panel used to develop the definition.