The recent emphasis on preparedness has created heightened expectations and has raised questions about the extent to which U.S. public health systems have evolved in recent years. This paper describes how public health preparedness is transforming public health agencies. Key signs of change include new partnerships, changes in the workforce, new technologies, and evolving organizational structures. Each of these elements has had some positive impact on public health; however, integration of preparedness with other public health functions remains challenging. The preparedness mission has also raised challenges in the areas of leadership, governance, quality, and accountability.