General aviation accounts for three-quarters of U.S. air traffic, from small propeller planes to large jets, operating among nearly 19,000 airports. While most security operations are left to private airport operators, the Transportation Security Administration (TSA), part of the Department of Homeland Security (DHS), provides guidance on threats and vulnerabilities. In 2004, TSA issued suggested security enhancements that airports could implement voluntarily. Unlike commercial airports, in most cases general aviation airports are not required to implement specific security measures. GAO was asked to perform onsite assessments at selected airports with general aviation operations to determine what physical security measures they have to prevent unauthorized access. With advance notice, GAO investigators overtly visited a nonrepresentative selection of 13 airports, based on TSA-determined risk factors. Three of the airports also serve commercial aviation and are therefore subject to TSA security regulations. Using TSA's voluntary recommendations and GAO investigators' security expertise, GAO determined whether certain security measures were in place. GAO also requested documentation of incidents of unauthorized access. Results of GAO's assessments cannot be projected to all general aviation airports and are not meant to imply that the airports failed to implement required security measures.