American workers increasingly rely on 401(k) plans for their retirement security, and sponsors of 401(k) plans--typically employers--have critical obligations under the Employee Retirement Income Security Act of 1974 (ERISA). When acting as fiduciaries, they must act prudently and solely in the interest of plan participants and beneficiaries. The Department of Labor (Labor) is responsible for protecting private pension plan participants and beneficiaries by enforcing ERISA. GAO examined: (1) common 401(k) plan features, which typically have important fiduciary implications, and factors affecting these decisions; (2) challenges sponsors face in fulfilling their fiduciary obligations when overseeing plan operations; and (3) actions Labor takes to ensure that sponsors fulfill their fiduciary obligations, and the progress Labor has made on its regulatory initiatives. To address these objectives, GAO administered a survey asking sponsors how they select plan features and oversee operations, reviewed industry research, conducted interviews, and reviewed related documents.