This book is about analog and digital filter design. The analog sections include both passive and active filter designs, a subject that has fascinated me for several years. Included in the analog section are filter designs specifically aimed at radio frequency engineers, such as impedance matching networks and quadrature phase all-pass networks. The digital sections include infinite impulse response (IIR) and finite impulse response (FIR) filter design, which are now quite commonly used with digital signal processors. Infinite impulse response filters are based on analog filter designs.