Reducing hospital readmission rates has become a priority to policymakers as readmissions are common, costly and, at least in theory, a reasonable fraction of them should be preventable. Policymakers therefore believe that reducing readmission rates represents a unique opportunity simultaneously to improve quality of care and reduce costs. As part of the Affordable Care Act (ACA), Congress directed the Centers for Medicare and Medicaid Services (CMS) to penalize hospitals with "worse than expected" 30-day readmission rates. This part of the ACA has motivated hospitals, health systems, professional societies and independent organizations to invest substantial resources into finding and implementing solutions for the "readmissions problem".