Complex numbers are extremely important in qEEG, because they are at the root of the Fourier’s theorem and related developments as coherence measurement. Obviously their applications exceed this field and their theory is far more widespread than this succinct introduction. Nevertheless, it is needed to briefly review the justification of the complex numbers to obtain square roots from negative numbers and its graphical representation by means of the Argand’s diagram. The polar form is presented. Finally, we introduce some basic concepts of algebra and the definition of complex functions.