At some point during 2009, more than 17 million households in the United States had difficulty providing enough food for all their members because of a lack of resources. In more than one-third of these households, the food intake of some household members was reduced and normal eating patterns were disrupted due to limited resources. The Workshop on Understanding the Relationship Between Food Insecurity and Obesity was held to explore the biological, economic, psychosocial, and other factors that may influence the relationship between food insecurity, overweight, and obesity in the United States. This report examines current concepts and research findings in the field. It identifies information gaps, proposes alternative approaches to analyzing data, recommends new data that should be collected, and addresses the limitations of the available research.