This chapter reviews the cellular mechanisms underlying the electroencephalogram (EEG) signal and the evidence of its correlation with higher-level brain functions. Thalamocortical oscillations responsible for theta-alpha rhythmic activities are due to alternating Ca2+/K+-dependent depolarizing/hyperpolarizing membrane potentials. A critical role in their activation is played by Ca2+ conductance (IT) that is inactive at resting membrane potential and de-inactivates at membrane potential more negative than 60 mV. Evidence of theta-alpha rhythm correlation with arousal, vigilance, and memory processes is reviewed. EEG activities in the narrowband gamma oscillations (30–80 Hz) are known to depend on synchronized activities of cortical parvalbumin-positive, fast-spiking, inhibitory, GABAergic interneurons using gamma-aminobutyric acid (GABA) as its neurotransmitter. Gamma rhythms have been shown to be significantly correlated with perceptual ongoing activities. Further EEG correlates of cognitive activities such as language, visual and auditory perception, memory, and learning are demonstrated by studies of brain connectivity which highlight the interest of EEG analysis in defining neural networks involved in higher brain functions.