This investigation has been undertaken to describe the development of the EEG in relation to age and sex. Children not possessing special signs or symptoms, which ex juvantibus imply a risk for the apperance of certain EEG patterns, in the literature looked upon as abnormal, have been investigated. EEG findings have thus been collected, which in clinical diagnostic work can be used as reference values in the evaluation of EEG in connection with different disease states in children. The investigation has been carried out as a transverse study and comprises EEG examinations on 743 selected children — 389 girls and 354 boys. The selection is based on the following criteria of normality: 1. An uneventful prenatal, perinatal, and neonatal period; 2. No disorders of consciousness (sporadic syncope, however, was accepted); 3. No head injury with cerebral symptoms; 4. No history of central nervous system diseases; 5. No obvious somatic disease; 6. No convulsions; 7. No family history of convulsive disorders other than those secondary to acquired cerebral damage; 8. No paroxysmal headache or abdominal pain; 9. No enuresis or encopresis after the fourth birthday; 10. No tics, stuttering, pavor nocturnus or excessive nailbiting; 11. No obvious mental diseases; 12. No conduct disorders; 13. No deviation with regard to mental and physical development. The children were collected from well-baby clinics, child care homes, nursery schools, and schools, all of which were situated in a part of the city with easy access by public transport to the laboratory. The past history of the children was controlled three times, and the delivery files were checked. A somatic examination was performed emphasizing neurological status. Handedness could be adequately judged from 3 years of age. The material has been described with regard to puberty, birth order in family and social family data. EEG was recorded at rest in all children, during hyperventilation in 82.6 per cent of them, during intermittent photic stimulation in 81.4 per cent, and during sleep in 80.5 per cent. After characterization of the different observed EEG patterns and description of statistical treatment, the results including significant correlations and some comments are presented. The most outstanding findings were those related to age and sex. The alpha frequency increased linearly with age, girls showing higher frequencies than boys. The amount of non-rhythmic low frequency activity was estimated, and the records were classified as having “minute”, “normal” (10–30%), “slightly increased”, and “moderately increased” amounts of low frequency activity always related to age. Up to about 8 years the incidence of records with slight increase of low frequency activity was higher in boys than in girls. In the 14 and 15 year-old children this condition was reversed. Rhythmic patterns in the resting EEG showed characteristic age and sex dependence. Slow posterior waves or polyphasic potentials were found more often in older children than in younger, and more often in girls than boys. Slow posterior rhythm or rhythmic 2.5—4.5 Hz posterior activity occurred with a maximal figure at 5 to 7 years of age and in this age group more often in girls than in boys. Diffuse (3-) 4—5 Hz activity during drowsiness without or with posterior accentuation occurred with a gradually declining incidence and was not observed after 11 or 8 years of age respectively. Without antecedent growth from slower rhythms present at an earlier age, two patterns were noticed — rhythmic 6—7 Hz anterior activity during drowsiness and mu rhythm. The first mentioned rhythm appeared mainly as a “male” trait with a maximal figure at 10—12 years of age, while the last mentioned rhythm was mainly a “female” trait with a successive increase in incidence throughout childhood. Responses to hyperventilation were characterized according to their location in anterior, posterior and diffuse rhythmic activities, as well as non-rhythmic low frequency activity in posterior derivations, which was the most common response. The anterior activity appeared from 9 years of age and the incidence increased significantly with age. The other activities showed a biphasic shape with a maximal incidence in ages around 9–12 years. Girls showed significantly more effects than boys. Responses to intermittent photic stimulation were characterized as increase of low frequency activity with a maximal figure between 5 and 9 years of age, and type of photic driving. Low frequency flicker responses were found more in younger than older children, while high frequency flicker responses were found more in older children. Girls responded to the activation significantly more than boys. Spontaneous sleep was achieved in 31% of all sleep-activated children. Despite barbiturate-induction 5% of the children could not fall asleep. In girls this finding was positively correlated to blood pressure. Maturational factors are suggested to be of basic importance for many of the EEG patterns found. To determine whether or not the recorded EEG findings represent normal EEG as they are recorded in normal children, a longitudinal as well as a psychological study in these children must be performed. The occurrence of paroxysmal activity and 14 and 6 Hz positive spike phenomenon in the children reported on in this investigation have been separately described. Ziel der vorliegenden Studie ist die Beschreibung der Entwicklung des EEG in Beziehung zu Lebensalter und Geschlecht. Es wurden Kinder untersucht, bei denen keine Zeichen oder Symptome vorhanden waren, die auf ein erhöhtes Risiko für das Auftreten von bestimmten, in der Literatur als abnorm angesehenen EEG-Mustern schließen ließen. Auf diese Weise wurden elektroenzephalographische Daten gesammelt, die in der klinischen Diagnostik, d. h. bei der...