We used data from the 1996-2005 Medical Expenditure Panel Survey to track changes in children's public insurance eligibility and coverage. During the 2001-2005 "postexpansion" period, eligibility was approximately constant, while public enrollment increased rapidly and uninsurance declined. Nevertheless, as of 2005, 62 percent of all uninsured children (5.5 million) continued to be eligible but not enrolled. We present detailed estimates of their characteristics by age, income, race/ethnicity, health status, and nativity/citizenship. We also examine the impact of potential changes in SCHIP income thresholds--both an expansion and a rollback--and estimate the number and characteristics of the children potentially affected.