The Pre-Existing Condition Insurance Plan (PCIP) was a national high-risk pool established under the Affordable Care Act (ACA) to provide coverage for individuals with preexisting conditions who had been uninsured for at least six months. It was intended to be a temporary program: PCIPs opened in 2010 and closed in April 2014. At that point, those with preexisting conditions could shop for health insurance in the marketplaces, where plans are prevented from using applicants' health status to deny coverage or charge more. This issue brief draws on the PCIP experience to outline why national high-risk pools, which continue to be proposed as policy alternatives to ACA coverage expansions, are expensive to enrollees as well as their administrators and ultimately unsustainable. The key lesson--and the principle on which the ACA is built--is that insurance works best when risk is evenly spread across a broad population.