Perhaps the single most important lesson to draw from this review is how much our estimates of the number and, particularly, the characteristics of uninsured children are affected by measurement error. Some of this error is widely acknowledged--such as the underreporting of Medicaid enrollment in surveys--but much of it is not. And even when the presence of error is recognized, this does not mean that researchers and policymakers know how to take it into account. We may know, for example, that Medicaid enrollment is underreported by 24 percent in a particular survey, but how does that affect the estimate of the uninsured? And how much does the apparent, substantial underreporting of Medicaid contribute to the perception that Medicaid is failing to reach millions of uninsured children? Until we can make progress in separating the measurement error from the reality of uninsurance, our policy solutions will continue to be inefficient, and our ability to measure our successes will continue to be limited.