A general procedure for creating Markovian interest rate models is presented. The models created by this procedure automatically fit within the HJM framework and fit the initial term structure exactly. Therefore they are arbitrage free. Because the models created by this procedure have only one state variable per factor, twoand even three-factor models can be computed efficiently, without resorting to Monte Carlo techniques. This computational efficiency makes calibration of the new models to market prices straightforward. Extended Hull- White, extended CIR, Black-Karasinski, Jamshidian's Brownian path independent models, and Flesaker and Hughston's rational log normal models are one-state variable models which fit naturally within this theoretical framework. The 'separable' n -factor models of Cheyette and Li, Ritchken, and Sankarasubramanian - which require n ( n + 3)/2 state variables - are degenerate members of the new class of models with n ( n + 3)/2 factors. The procedure is used to create a new class of one-factor models, the ' β- -models.' These models can match the implied volatility smiles of swaptions and caplets, and thus enable one to eliminate smile error. The β- -models are also exactly solvable in that their transition densities can be written explicitly. For these models accurate - but not exact - formulas are presented for caplet and swaption prices, and it is indicated how these closed form expressions can be used to efficiently calibrate the models to market prices.