This document summarizes the current status of immunization information systems (IIS) in the United States. It finds that as of 2007, 71% of children under 6 years old had their immunization records in an IIS. While most states authorize or mandate IIS reporting, participation levels among providers vary significantly. The document also examines IIS capabilities for tracking adolescent and adult immunizations as well as performance measures like data timeliness and security policies. Overall IIS participation has increased in recent years but opportunities remain to improve coverage and functionality.