Pandemic response takes place in distributed, uncertain, and high-tempo environments. These conditions require public health agencies to rapidly generate and roll out publicly accountable responses in the face of incomplete and ambiguous evidence. To perform under these conditions, public health organizations have devised several tools to support decision making and response. This article examines two such tools that debuted during the 2009 H1N1 outbreak—the 2005 International Health Regulations and influenza pandemic planning. Relying on an international network of researchers who gained access to lead public health agencies in advance of the 2009 pandemic, this study draws on several forms of data—primary documentation, interviews, and an extended workshop with key officials—that were collected as the pandemic unfolded. With this unique dataset, we analyze the performance of the International Health Regulations and pandemic influenza plans from a "sensemaking" perspective. We find that insufficient attention to both the complexities and time horizons involved with adequate sensemaking limited the ability of both tools to fully meet their goals. To improve organizational performance during global pandemics, the sensemaking perspective calls attention to the importance of informal venues of information- sharing and to the need for decisionmakers to continually update planning assumptions.