The Employee Benefits Security Administration (previously known as Pension and Welfare Benefits Administration)
The Employee Benefits Security Administration is responsible for administering and enforcing the fiduciary, reporting and disclosure provisions of Title 1 of the Employee Retirement Income Security Act of 1974.
The Internal Revenue Service has "primary responsibility for participation, vesting and funding issues of ERISA." Title II of ERISA (which amended the Internal Revenue Code) is administered by the IRS.
The Pension Benefit Guaranty Corporation is a federal government corporation established by Title IV of the Employee Retirement Income Security Act of 1974 (ERISA) to protect "the retirement incomes of about 44 million American workers in about 32,500 privately defined benefit pension plans." Annual Reports (from 1995) are available in pdf format.