Currently U.S. firms hold record amounts of cash. The authors explore cross-sectional variation in cash holdings of U.S. publicly traded firms to shed light on the reasons for this recent trend. First, they identify factors that correlate with cash holdings and then examine the evolution of these factors over the past decade. Several factors, including research and development expenditures and idiosyncratic uncertainty, are important in accounting for cross-sectional differences in cash holdings. However, these factors do not increase over time as cash holdings do; thus, it seems unlikely that they underlie the increase in cash holdings. Aggregate uncertainty, however, has recently reached record levels. This uncertainty, combined with the fact that (idiosyncratic) uncertainty correlates well with cash holdings in the cross section of firms, suggests aggregate uncertainty may be an important factor accounting for the recent trend in increased cash holdings.