The P300 brain potential can provide information about cognition that is quantitatively comparable to other clinically used biomedical assays. Causes of P300 variability with respect to task and biologic determinants have been well characterized so that refinement of ERP methods for clinical applications is possible. Elaboration of how P300 and other ERP components reflect neuropsychologic processes would help to increase their clinical relevance. In particular, development of reliable P3a paradigms used in conjunction with P3b tasks promises to augment dramatically the applicability and sensitivity of ERPs. Use of P300 as a clinical evaluation tool should be revisited with contemporary theory, methods, and analysis procedures because a reliable neuroelectric measure of mental function would redefine the assessment of cognitive disorders.