In the previous chapter Fourier series has been introduced. To study aperiodic functions, Fourier theorem is extended to another technique called Fourier transforms in which the period of a given pulse is increased so that the fundamental frequency becomes zero. Hence, the spacing between the harmonics also becomes zero and the frequency distribution does not become zero. Fourier series becomes Fourier transforms. The frequency distribution of harmonics in the case of Fourier series is discrete whereas it is continuous in the case of Fourier transforms. The main aim of this chapter is to introduce Fourier transforms.