In this book, an effective method for designing planar microstrip filters by using Continuously Varying Transmission Line (CVTL) theory is presented. This method comprises of two parts. The first part is to find the mathematical expressions of the scattering parameters of CVTL. The second part is to apply an optimization technique to find the suitable values that will match the specific desired frequency response. Genetic Algorithm is the optimization technique utilized in this book due to its robustness and efficiency. The aim of this book is to find the impedance profile and the length of CVTL that will satisfy the desired frequency response whether it is low pass, stop band, band pass or high pass filter. Accordingly, the width of the microstrip will be determined once the impedance profile is known. The advantage of this technique is that the filter impedance profile is continuous which minimizes the internal power reflection.