Inadequate insurance, whether private or public, can impose ruinous private hardship and unexpected public burdens. "Who are the underinsured?" is both a definitional and an empirical question. Data from the National Medical Care Expenditure Survey, applied under various concepts of risk and expense, reveal that over a quarter of the nonelderly population is inadequately protected against the possibility of large medical bills. The private burdens of the underinsured are widely distributed across the population.