This chapter presents the electrophysiological findings, to date, in adult attention-deficit hyperactivity disorder (ADHD). It provides a lifespan perspective of the electrophysiology of ADHD; these findings in adults are discussed in relation to those in children. The electroencephalography (EEG) and event-related potential (ERP) findings in children, adolescents, and adults with ADHD suggest that electrophysiological indices represent underlying processes that are developmentally stable. Electrophysiological assessments enable measurement of covert processes, and the superior temporal resolution enables precise tracking of different steps in information processing, which is critical for ADHD theory. The functional ERP findings indicate that patterns of abnormal processing, firmly established in childhood ADHD, persist in adult ADHD, particularly preparatory and inhibitory processes. These findings provide external validation of the ADHD diagnosis in adults, which is becoming increasingly recognized as a common psychiatric disorder in adulthood.