The developmental and clinical aspects in the literature on triple X syndrome are reviewed. Prenatal diagnosis depends on karyotyping. The incidence is 1 of 1000 females. At birth, 47,XXX girls have a lower mean birth weight and a smaller head circumference. Triple X diagnosis was not suspected at birth. The maternal age seems to be increased. Toddlers with triple X syndrome show delayed language development. The youngest girls show accelerated growth until puberty. EEG abnormalities seem to be rather common. Many girls show motor-coordination problems and auditory-processing disorders are not rare. Scoliosis is probably more common in adolescent cases. The IQ levels are 20 points below that of controls, and verbal IQ is lowest. The girls struggle with low self-esteem and they need psychological, behavioural and educational support. They perform best in stable families. After leaving school they seem to feel better. In adults, premature ovarian failure seems to be more prevalent than in controls. MRIs of the brain seem to show decreased brain volumes. The 47,XXX women most often find jobs that reflect their performance abilities. Psychotic illness seems to be more prevalent in triple X adult women than in controls. Psychotic disorders respond well to psychotropic drugs. Triple X adults suffer more frequently from cyclothymic and labile personality traits. Research on triple X syndrome may yield more insight into brain and behaviour relations, developmental psychopathology, auditory-processing disorders, EEG disorders, personality and psychotic disorders, etc.