The Pension Benefit Guaranty Corporation (PBGC) was established by the Employee Retirement Income Security Act of 1974 to ensure that participants in defined benefit pension plans receive their pensions if their plans terminate without sufficient assets to pay promised benefits. PBGC administers separate insurance programs to protect participants in single-employer and multiemployer plans. PBGC has published the Pension Insurance Data Book annually since 1996 to present detailed statistics on PBGC program operations and benefit protection. In addition to tabular presentations of current and historical data on PBGC's single-employer and multiemployer pension insurance programs, this edition features graphics illustrating selected current data and trends in PBGC operations and in the U.S. private pension universe insured by PBGC. A number of tables in this Data Book are based on data from the 1999 Form 5500. Problems with the processing of these forms resulted in incomplete data. When data were missing, we substituted data from the 1998 Form 5500.