This chapter illustrates by means of a P300 virtual keyboard application, several fundamental notions of brain–computer interfaces (BCIs): preprocessing, extraction of characteristics, classification and training. It proposes to the reader to appropriate them with a specific application. The P300 virtual keyboard application flashes the symbols of a virtual keyboard displayed on a screen. When a flash appears on the symbol that the user wants to select target, their brain activity presents a particular response involving several neurophysiological phenomena. The real-time analysis of nerurophysiological signals and their classification allows detecting the presence or absence of the P300 wave in response to the flashes, and in fine to determine the symbol that the user wants to select. The chapter explains the binary classification in the specific context of the P300 wave and exposes the implementation of the processing chain in OpenViBE as well as the totality of the software system that controls the P300 virtual keyboard.