This review article summarizes some recent developments in psychiatry such as personalized medicine, employing biomarkers and endophenotypes, and developments collectively referred to as neuromodulation with a focus on ADHD. Several neurophysiological subtypes in ADHD and their relation to treatment outcome are reviewed. In older research the existence of an "abnormal EEG" or "paroxysmal EEG" was often reported, most likely explained by the high occurrence of this EEG subtype in autism, as the diagnosis of autism was not coined until 1980. This subgroup might respond best to anticonvulsant treatments, which requires more specific research. A second subgroup is a beta-excess or beta-spindling subgroup. This group responds well to stimulant medication, albeit several studies suggesting that neurophysiologically this might represent a different subgroup. The third subgroup consists of the "impaired vigilance" subgroup with the often-reported excess frontal theta or excess frontal alpha. This subgroup responds well to stimulant medication. Finally, it is proposed that a slow individual alpha peak frequency is an endophenotype related to treatment resistance in ADHD. Future studies should incorporate this endophenotype in clinical trials to further investigate new treatments for this substantial subgroup of patients, such as NIRS-biofeedback, transcranial Doppler sonography biofeedback, hyperbaric oxygen therapy, or medications such as nicotine and piracetam.