Term-structure models are widely used to price interest rate derivatives, such as swap options and bonds with embedded options. We describe how a general one-factor model of the short rate can be implemented as a recombining trinomial tree and calibrated to market prices of actively traded instruments. The general model encompasses most popular one-factor Markov models as special cases. The implementation and the calibration procedures are sufficiently general that they can select the functional form of the model that best fits the market prices. This characteristic allows the model to fit the prices of in- and out-of-the-money options when there is a volatility skew. It also allows the model to work well with economies characterized by very low interest rates, such as Japan, for which other models often fail.