Endogenous potentials are evoked responses that occur independent of the stimulus evoking them. They are related to different aspects of information processing. Two types of event related potentials (ERPs) are recorded, the P300 wave and contingent negative variation (CNV). The P300 wave is the most frequently studied factor in various clinical diseases and also for neurological research. Different tasks, paradigms can be used of which the oddball paradigm, consisting of target and non-target stimuli and their variations is most popular. The effect on P300 of physiological factors like age, IQ, pregnancy, exercise and others along with the clinical application in various neurophysiological and psychiatric disorders are discussed. This review deals with various aspects of (ERPs), their methodology with a broad view of the scope and clinical horizon as regard their application in neurophysiological research.