The potential for interest rates to rise, which would cause bond values to fall. Bond prices and interest rates are inversely linked, as if on opposite ends of a seesaw: if one rises, the other falls. The longer the maturity date, the greater the swing in prices. (a 30-day T‑bill would sit at the center of a seesaw and thus experience little change in price due to changes in interest rates, while a 30-year T‑bond would sit at the far end of the seesaw and thus experience far greater swings in price.) Thus, if you sell a bond prior to maturity, the price you receive may be more or less than the price you paid to buy it, depending on what the interest rates were when you bought the bond compared to current rates. See Coupon, Duration, Interest, Yield, Zero-Coupon Bond