This chapter describes electrophysiological activities whose variations, when recorded by a surface electroencephalogram (EEG), are analyzed in order to extract a marker. Variation in EEG signals can be extracted either directly in the spatiotemporal domain, or after a frequency transformation. Spatiotemporal analysis is used to detect an event-related potential (ERP), for which there is a correlation between the phase of oscillations appearing as a response to an event and the instant at which it occurs. The chapter describes a marker for an activity evoked by an endogenous event that has been widely used to implement BCIs, namely slow variations in average cortical potential. It presents a marker related to the realization of a motor imaging task, namely bereitschaftspotential (BP) or readiness potential. The chapter describes the markers of an activity evoked by an exogenous event, in this case a stimulation emitted by the BCI: steady-state somatosensory evoked potentials, visual (SSVEP) and cognitive (P300) evoked potentials.