Event-related potentials (ERPs) are voltage fluctuations of the brain that are time-locked to some physical or mental occurrence. These potentials can be recorded safely from the human scalp and provide valuable information about the brain activity associated with perception, cognition, and behavior. This paper deals with theoretical issues concerning the use of ERPs in psychological research. Following a brief description of the basic method of ERP recording, technical problems that can occur in the recording and analysis of the ERP were discussed (e.g., ocular artifacts, averaging, and individual differences). Then, the problems of isolating and interpreting ERP components were addressed. Finally, the relationship between the ERP and other psychological measures was discussed in the light of functional systems of the brain. It was concluded that the ERP can serve as a valuable tool in psychological research only if it is combined with other psychological measures.