Frequency-domain analysis is by far the most widely used numerical method in the analysis of neurophysiological signals, especially in clinical practice. The fundamental method is Fourier transform analysis. This method allows an EEG signal to be decomposed into its spectral components, that is, into pairs of sine and cosine functions for a given frequency. The set of all these terms, as a function of frequency, is called the power spectrum (PS), and it allows for the identification of frequency components in an easy and intuitive manner. Given the importance of this method, we will analyze in detail to understand its rationale. We will see how Fourier series analysis for a periodic function defined with a period of 2π can be extended to non-periodic functions using the Fourier transform function. We will also examine the discrete version of the transform. Other frequency-domain analysis methods will also be discussed, such as density spectral array (DSA) and wavelet analysis.