Book excerpt: When policy makers and researchers consider potential solutions to the crisis of uninsurance in the United States, the question of whether health insurance matters to health is often an issue. This question is far more than an academic concern. It is crucial that U.S. health care policy be informed with current and valid evidence on the consequences of uninsurance for health care and health outcomes, especially for the 45.7 million individuals without health insurance. From 2001 to 2004, the Institute of Medicine (IOM) issued six reports, which concluded that being uninsured was hazardous to people's health and recommended that the nation move quickly to implement a strategy to achieve health insurance coverage for all. The goal of this book is to inform the health reform policy debateâ€"in 2009â€"with an up-to-date assessment of the research evidence. This report addresses three key questions: What are the dynamics driving downward trends in health insurance coverage? Is being uninsured harmful to the health of children and adults? Are insured people affected by high rates of uninsurance in their communities?