Across the United States, millions of Americans volunteer their time and efforts to improve the social fabric of their communities. Inevitably, some of these volunteers will be medical and public health professionals. However, because of the complexities of the health field, including concerns about credentialing, training and legal protections, many of these persons have not been able to volunteer in their professional capacities. The terrorist events of 2001 showed that not only would individuals with medical and public health expertise want to volunteer, but that their help could be very much needed in future mass catastrophic events. The Medical Reserve Corps Program was created as a national system of community-based units to promote the local identification, recruitment, training, and activation of volunteers, especially those with medical and public health backgrounds. These Medical Reserve Corps units supplement the existing public health and emergency response entities in the community.