We document a dramatic increase from 1980 through 2006 in the average cash ratio for U.S. firms. We show that this increase is concentrated among firms that do not pay dividends, firms in more recent IPO listing cohorts, and firms in industries that experience the greatest increase in idiosyncratic volatility. After documenting the increase in cash ratios, we investigate the causes for that increase. We use a model for cash holdings developed by Opler, Pinkowitz, Stulz, and Williamson (1999), but recognize that the literature has not made enough progress to provide a model that dominates all others. We find that the main reasons for the increase in the cash ratio are that inventories have fallen, cash flow risk for firms has increased, capital expenditures have fallen, and R&D expenditures have increased. While the contribution of changes in these firm factors to the overall increase in cash holdings varies across alternative empirical models of cash holdings, our conclusions are generally robust.