There are currently a number of theories relating to the psychological correlates of the P300. Many of these have been derived from complex relations that have been observed between P300 attributes and such stimulus parameters as target probability, number of preceding non-targets, and inter-stimulus interval (within oddball-type paradigms). However, recent evidence suggests that a number of these relations may be more economically explained by the varying target-to-target intervals employed in those studies, which in turn may have significant theoretical and clinical implications including a re-examination of the relative importance of the P300’s potential physiological and cognitive determinants. This presentation will describe some of the research in support of the target-to-target interval explanation of the P300, including data from both auditory and visual paradigms, as well as some preliminary data from pharmacological manipulations within non-clinical samples.