International Polar Year 2007-2008 (IPY) was an intense, coordinated field campaign of observations, research, and analysis. It was the largest, most comprehensive campaign ever mounted to explore Earth's polar domains. Legacies and Lessons of the International Polar Year 2007-2008 summarizes how IPY engaged the public to communicate the relevance of polar research to the entire planet, strengthened connections with the Indigenous people of the Arctic, and established new observational networks. Legacies and Lessons of the International Polar Year 2007-2008 also addresses the objectives articulated for IPY in the 2004 National Research Council report, A Vision for International Polar Year (NRC, 2004). These objectives include: suggestions for scientific communities and agencies to use the IPY to initiate a sustained effort aimed at assessing large-scale environmental change and variability in the polar regions, the need to explore new scientific frontiers from the molecular to the planetary scale, investment in critical infrastructure and technology to guarantee that IPY 2007-2008 leaves enduring benefits for the nation and for the residents of northern regions, as well as increase public understanding of the importance of polar regions in the global system. Legacies and Lessons of the International Polar Year 2007-2008 explains how activities at both poles led to scientific discoveries that provided a step change in scientific understanding and helped translate scientific knowledge into policy-relevant information. At a time when the polar regions are undergoing a transformation from an icy wilderness to a new zone for human affairs, these insights could not be more timely or more relevant. From outreach activities that engaged the general public to projects that brought researchers from multiple disciplines and several nations together, the legacies of IPY extend far beyond the scientific results achieved, and valuable lessons learned from the process will guide future endeavors of similar magnitude.