Neurophysiology allows us to understand and modulate the neural mechanisms in ADHD with high time- and/or frequency-resolution. These non-invasive methods include electroencephalographic recordings at rest and during tasks, with spontaneous and event-related oscillations and potentials tracking covert processing and transcranial neuromodulation through magnetic or electric fields. The findings indicate consistent cognitive and neural deficits in ADHD related to impaired attention and deficient inhibition. Advanced signal processing and source imaging methods often converge with other imaging approaches. Neurophysiological findings also reveal considerable heterogeneity in ADHD regarding cognitive, affective, and genetic subtypes. This illustrates the importance of dimensional approaches and of pathophysiological mechanisms partly shared with other disorders. Although several potential neurophysiological markers of ADHD have been considered, a clinical use for individual diagnostics and classification is not supported to date. More research should clarify the clinical potential of multivariate multimodal classification and prediction of treatment outcome to advance individualized treatment.