Several key events during the financial crisis underscored the vulnerability of the financial system to systemic risk. One such event was the September 2008 run on money market funds (MMFs), which began after the failure of Lehman Brothers Holdings, Inc., caused significant capital losses at a large MMF. Amid broad concerns about the safety of MMFs and other financial institutions, investors rapidly redeemed MMF shares, and the cash needs of MMFs exacerbated strains in short-term funding markets. These strains, in turn, threatened the broader economy, as firms and institutions dependent upon those markets for short-term financing found credit increasingly difficult to obtain. Forceful government action was taken to stop the run, restore investor confidence, and prevent the development of an even more severe recession. Even so, short-term funding markets remained disrupted for some time. The Treasury Department proposed in its Financial Regulatory Reform: A New Foundation (2009), that the President's Working Group on Financial Markets (PWG) prepare a report on fundamental changes needed to address systemic risk and to reduce the susceptibility of MMFs to runs. Treasury stated that the Securities and Exchange Commission's (SEC) rule amendments to strengthen the regulation of MMFs-which were in development at the time and which subsequently have been adopted-should enhance investor protection and mitigate the risk of runs. However, Treasury also noted that those rule changes could not, by themselves, be expected to prevent a run on MMFs of the scale experienced in September 2008. While suggesting a number of areas for review, Treasury added that the PWG should consider ways to mitigate possible adverse effects of further regulatory changes, such as the potential flight of assets from MMFs to less regulated or unregulated vehicles. This report by the PWG responds to Treasury's call.1 The PWG undertook a study of possible further reforms that, individually or in combination, might mitigate systemic risk by complementing the SEC's changes to MMF regulation. The PWG supports the SEC's recent actions and agrees with the SEC that more should be done to address MMFs' susceptibility to runs. This report details a number of options for further reform that the PWG requests be examined by the newly established Financial Stability Oversight Council (FSOC). These options range from measures that could be implemented by the SEC under current statutory authorities to broader changes that would require new legislation, coordination by multiple government agencies, and the creation of new private entities. For example, a new requirement that MMFs adopt floating net asset values (NAVs) or that large funds meet redemption requests in kind could be accomplished by SEC rule amendments. In contrast, the introduction of a private emergency liquidity facility, insurance for MMFs, conversion of MMFs to special purpose banks, or a two-tier system of MMFs that might combine some of the other measures likely would involve a coordinated effort by the SEC, bank regulators, and financial firms.