This book provides a detailed description of some of the most widely used rational fitting techniques for approximation of frequency domain responses. The techniques are: Bode’s asymptotic approximation, the Levy method, iteratively reweighted least squares, the Sanathanan-Koerner method, the Noda method, Vector Fitting, the Levenberg-Marquardt method, and the Damped Gauss-Newton method. A MATLAB routine for each technique is presented. These techniques are tested by approximating synthetic frequency domain responses. Then, they are applied to the rational approximation of the frequency-dependent parameters corresponding to a single-phase transmission line. The effect of the rational function-based models is evaluated, considering transients in three cases: Open-ended, short-circuited, and perfectly matched lines. The error levels obtained in time domain simulations are consistent with the fitting deviations of the frequency-dependent parameters. The book concludes by showing main advantages and disadvantages for each technique.