This book enables readers to become familiar with passive and active realizations of the impedances and important filter transfer functions (TFs). Firstly, time, s, and frequency domain analysis of the R-L, R-C, and R-L-C circuits are discussed. Then, active realizations of the impedances and TFs using different active devices are introduced. Readers will benefit from an example-driven approach to topics such as operational amplifiers, current followers, voltage followers, unity gain inverting amplifiers, negative impedance converters, first-generation current conveyors, second-generation current conveyors, third-generation current conveyors, differential voltage current conveyors, etc. will be introduced, which will be accompanied with a number of examples. Non-ideal gain effects on the performance of the active circuits are also demonstrated.