This article examines the sociodemographic and health characteristics of the underinsured-people who have some health insurance but are having trouble paying for health care or medications. It uses data from the Boston Area Community Health (BACH) Survey, a large (N=5503) community-based random sample of Boston residents aged 30 to 79 years (1767 black, 1,877 Hispanic, and 1859 white; 2301 men and 3202 women). The authors found that minorities were less likely than whites to have health insurance (for men and women, respectively, 30% and 19% of Hispanics, 16% and 9% of blacks, and 9% and 7% of whites lacked health insurance). Blacks were the most likely to be underinsured (for men and women, respectively, 18% and 20% of blacks vs. 9% and 14% of Hispanics and 8% and 12% of whites were underinsured). Those of lower and middle socioeconomic status were also more likely to be uninsured or underinsured. The health status of the uninsured was similar to that of the adequately insured, whereas those who were underinsured reported more co-morbidities and depression. The underinsured are generally older and sicker, and make greater use of the health care system, and may present a larger public health and health policy challenge than the uninsured.