The purpose of this chapter is to familiarize the reader with the basic electrical patterns of the electroencephalogram (EEG). Brain cells (mainly neurons and glia) are organized in multiple levels of intricate networks. The cellular membranes are semipermeable media between extracellular and intracellular solutions, populated by ions and other electrically charged molecules. This represents the basis of electrical currents flowing across cellular membranes, further generating electromagnetic fields that radiate to the scalp electrodes, which record changes in the activity of brain cells. This chapter presents these concepts together with the mechanisms of building up the EEG signal. The chapter discusses the various behavioral conditions and neurophysiological mechanisms that modulate the activity of cells leading to the most common EEG patterns, such as the cellular interactions for alpha, beta, gamma, slow, delta, and theta oscillations, DC shifts, and some particular waveforms such as sleep spindles and K-complexes and nu-complexes.