401(k) plan sponsors have increasingly offered participants managed accounts-services under which providers manage participants' 401(k) savings over time by making investment and portfolio decisions for them. These services differ from investment options offered within 401(k) plans. Because little is known about whether managed accounts are advantageous for participants and whether sponsors understand their own role and potential risks, GAO was asked to review these services. GAO examined (1) how providers structure managed accounts, (2) their advantages and disadvantages for participants, and (3) challenges sponsors face in selecting and overseeing providers. In conducting this work, GAO reviewed relevant federal laws and regulations and surveyed plan sponsors. GAO interviewed government officials, industry representatives, other service providers, and 12 plan sponsors of varying sizes and other characteristics. GAO also conducted case studies of eight managed account providers with varying characteristics by, in part, reviewing required government filings.