Iezzoni and Freedman discuss the importance of defining disability in preventing or deterring disability. Forty million to 50 million individuals in the US now live with potentially disabling conditions. Although rates of some serious limitations among elderly individuals have declined, sobering reports warn of higher rates of potentially impairing conditions among children and working-age adults. These latter trends are multifaceted with diverse contributors, including major therapeutic breakthroughs that now save lives of severely impaired individuals who would once have died and increasing prevalence of overweight and obesity among youth and young adults, along with associated problems such as diabetes.