This report is based on the General Accounting Office's audit at 6 of the 16 category X airports, an analysis of the Federal Aviation Administration's security inspection reports for all 16 airports, and a review of the reports on domestic aviation security by the Department of Transportation's Safety Review Task Force. Findings indicate security deficiencies among category X airports. The chief deficiencies involved ineffective passenger screening and inadequate controls over personnel identification systems and over access to those parts of the airport where aircraft operate. The report recommends inventorying personnel identification badges to determine the number not accounted for and to ensure the development of controls over identification systems. Airline and airport employees should be trained to challenge the presence of unauthorized persons in work areas. The Federal Aviation Administration should also assess the extent to which individual airlines should be permitted to exempt employees from the passenger screening process.