The relationship between the P3a and P3b event-related brain potentials (ERPs) will be reviewed by outlining the empirical background of the distinction between these subcomponents. Recent results suggest that the key factors for eliciting P3a in a three-stimulus oddball task are salience rather than novelty of the distracter stimulus and how target/standard discrimination difficulty modulates task processing. Although the exact neural loci of P3a and P3b generation are still unclear, discriminating a target from a standard stimulus appears to initiate frontal engagement as a consequence of the focal attention engaged by task demands. When this attentional focus is disrupted, P300 generation may then originate from an interaction between frontal lobe (P3a) and hippocampal/temporal-parietal (P3b) activity. This overall approach removes the involvement of novelty as a requirement for P3a production and opens the door for a more direct P300 theory.