in the world. As of Sep- tember 2001, its total marketable securities outstanding totaled $3.339 trillion. Of this total, $734.86 billion represents Treasury bills.1 Treasury bills are short-term discount instruments with original maturi- ties of less than one year. All Treasury securities are backed by the full faith and credit of the U.S. government. This fact, combined with their volume (in terms of dollars outstanding) and liquidity, afford Treasury bills a central place in the money market. Indeed, interest rates on Trea- sury bills serve as benchmark short-term rates throughout the U.S. econ- omy as well as in international money markets. This chapter provides an in-depth treatment of Treasury bills. We will describe the types of Treasury bills, how they are auctioned, price and yield calculations, and how the secondary market is organized. We will also discuss the time series behavior of Treasury bill yields relative to other key money market rates. Finally, we will discuss one time-tested portfolio strategy using Treasury bills-riding the yield curve.       TYPES OF TREASURY BILLS   Treasury bills are issued at a discount to par value, have no coupon rate, and mature at par value. Currently, the Treasury issues four types of Trea- sury bills that vary by their original maturity-28 day (1-month), 91 day (3-month), 182 day (6-month), and cash management bills.2As discussed in the next section, 1-month, 3-month, and 6-month bills are offered for sale each week.   1Source: Treasury Bulletin. 2ThefirstsixdigitsoftheCUSIPforaTreasurybillare"912795."   23     Cash management bills are offered from time to time with various maturities. The time between the announcement of an issue, auction, and