Insurance inherently involves collective destinies among members of a community -- a sharing of the risk of loss from untoward events. This, the fourth report from the Institute of Medicine's Committee on the Consequences of Uninsurance, explores a mirror image of the communal fortunes of those who are insured: the implications for a community of people who share geography and health services when not all of them are insured. It highlights the critical question of what happens to the broader community when the health insurance risk pool is highly fragmented and exclusionary. The report identifies an array of potential societal consequences of uninsurance, beyond the well-known implications for individual persons, families, and uninsured persons as a group. It reviews data on the association between uninsurance levels and community-wide measures of access to care, economic and social conditions, and overall health status; its sources range from sophisticated commissioned studies on uninsurance, hospital services, and financial margins to case studies and anecdotes. The Committee on the Consequences of Uninsurance finds that the adverse effects of uninsurance on the uninsured and the associated financial strain have spillover effects on health care institutions and providers.