This paper examines measures of Medicaid and Childrens Health Insurance Program (CHIP) coverage duration available for potential inclusion in a core set of children's health care quality measures as called for by the Children's Health Insurance Program Reauthorization Act (CHIPRA) of 2009. The authors review possible measures and argue that prospective measures of duration and churning based on cohorts of new enrollees hold the most promise for producing valid estimates and that additional technical and financial resources and enhancements to administrative data systems will be needed to support state efforts in this area of quality assessment.