Humans obtain information from various kinds of stimuli. The perception of these external stimuli may be modulated by the ongoing pre-stimulus brain activities. Multiple cognitive functions including arousal, attention (spatial and feature-based) and mental imagery may start prior to stimulus onset. Different roles of pre-stimulus oscillations in these cognitive functions, including gating by inhibition via pre-stimulus alpha signals, noise clearance, large-scale brain synchronizations, and motor preparation are discussed. We will also describe the effect of pre-stimulus explicit and implicit expectation on later processing and the way it is coupled with pre-stimulus attention. Finally, we will explain the temporal organization of pre-stimulus oscillations and their modulatory effect on the perception of the incoming stimulus. We will present current evidence concerning these different cognitive connotations and discuss present controversies.