Congress created the State Children's Health Insurance Program (SCHIP) to reduce the number of uninsured children in low-income families that do not qualify for Medicaid. States have flexibility in structuring their SCHIP programs, and their income eligibility limits vary. Concerns have been raised that individuals might substitute SCHIP for private health insurance--known as crowd-out. GAO was asked to examine the Centers for Medicare & Medicaid Services' (CMS) and states' efforts to minimize crowd-out and determine whether it should be a concern. GAO examined (1) CMS's guidance to states for minimizing crowd-out and assessment of whether it should be a concern and (2) states' policies to minimize crowd-out and how they assess whether it should be a concern. To do the work, GAO reviewed federal laws and guidance, examined state annual reports, and interviewed CMS officials. GAO also interviewed SCHIP officials from nine states. To improve information on whether crowd-out should be a concern in SCHIP, GAO recommends that the Acting Administrator of CMS ensure that states (1) collect and report consistent information on the extent to which SCHIP applicants have private insurance available to them and (2) take appropriate steps to determine whether available private health insurance is affordable for SCHIP applicants. (Contains 5 tables, 5 figures, and 55 footnotes.)