The literature on the relationship between background EEG and event-related brain potentials (ERPs) is reviewed, with the conclusion that variation in the former can contribute to individual variability in the latter. The effects of background EEG activity on the P300 component are then described using the results of three experiments. Study 1 assayed the association between EEG spectral power/mean frequency and P300 amplitude/latency measures in young adults. For the slower delta, theta, and alpha bands generally strong correlations who obtained for both types of measures. Study 2 employed similar techniques to assess a large sample of adults who varied in age from 20-80+ years. EEG power in the slower bands was correlated positively with P300 amplitude across the age range, but few effects for mean frequency/component latency were observed. Study 3 measured a group of young adults ten times very 20 min to assess for temporal changes in the relationship between EEG and ERPs. The correlations between spectral power and P300 amplitude measures were found to vary in a manner that suggested the influence of ultradian rhythms on neuroelectric activity. Taken together, the findings from all three study indicate that background EEG variation contributes significantly to the individual variability of the P300 ERP. Theoretical and applied implications of the findings are discussed.