Forty-six million people, or nearly one in five nonelderly adults and children, currently lack health insurance, an increase of 6 million since 2000. Developing effective strategies for reducing uninsurance requires understanding why people lack insurance coverage. This brief looks at the reasons people report being uninsured. We find that the high cost of health insurance matters for uninsured nonelderly adults and children, whether old or young, healthy or disabled, with high incomes or incomes below the poverty level. Further, the importance of high costs as a reason for being uninsured has risen rapidly, growing steadily for nonelderly adults and children.