This article studies the mid-September 2019 stress in U.S. money markets: On September 16 and 17, unsecured and secured funding rates spiked, and on September 17, the effective federal funds rate broke the ceiling of the Federal Open Market Committee (FOMC) target range. We highlight two factors that may have contributed to these events. First, reserves may have become scarce for at least some depository institutions, in the sense that these institutions’ reserve holdings may have been close to, or lower than, their desired level. Moreover, frictions in the interbank market may have prevented the efficient allocation of reserves across institutions, so that although aggregate reserves may have been higher than the sum of reserves demanded by each institution, they were still scarce given the market’s inability to reallocate reserves efficiently. Second, we provide evidence that some large domestic dealers likely experienced an increase in intermediation costs, which led them to charge higher spreads to ultimate cash borrowers. This increase was due to a temporary reduction in lending from money market mutual funds, including through the Fixed Income Clearing Corporation’s sponsored repo program.