Publisher Summary This chapter discusses the frequency domain circuit and signal analysis, based on the application of Fourier techniques, with special emphasis on the use of these techniques in the context of circuits. It begins by discussing sinusoidal alternating current (AC) signals and circuits. Phasor analysis is presented and it shows that this greatly simplifies this analysis because it allows the introduction of the “generalized impedance.” The phasor analysis technique is extended to analyze circuits driven by non-sinusoidal signals. This is done by first discussing the Fourier series that presents periodic signals as a sum of phasors. The Fourier series is a very important tool that forms the basis of fundamental concepts in signal processing such as spectra and bandwidth. The chapter concludes with the Fourier transform that allows the analysis of virtually any time-varying signal in the frequency domain.