In our study of electric circuits so far, we have primarily focused on circuits driven by sinusoidal sources. However, many real-world signals encountered in electrical engineering are non-sinusoidal, yet often exhibit periodic behavior. This chapter introduces the powerful tool of Fourier Series, which allows us to represent such periodic signals as a sum of sinusoids of different frequencies. Building upon this foundation, we will then explore the Fourier Transform, a technique that extends this concept to analyze non-periodic signals in the frequency domain. Understanding these concepts is crucial for analyzing the behavior of circuits subjected to complex waveforms and for designing systems that operate across a spectrum of frequencies.