Klinefelter syndrome is the most common chromosome abnormality in humans. The estimated prevalence is one in 500 to one in 1000 males but due to the widely variable and often aspecific features, only one in four cases are recognized. The most specific clinical features which can be observed at adult age are small testes, gynecomastia, female distribution of fat and body hair, slightly increased body length due to an increased leg length and azoospermia. Cognition is characterized by verbal deficits and psychosocial features include autistiform behavior. Structural brain abnormalities have been observed by MRI, such as decreased brain volumes and a decrease of asymmetry in areas corresponding to language performance. In the vast majority of cases a non-mosaic 47,XXY karyotype is observed. Parental imprinting of the extra X chromosome, variable inactivation of some X-chromosomal genes and CAG repeat length polymorphism of the androgen receptor may all be related to the variability of the phenotype. Surgical procedures of obtaining sperm in combination with repeated intracytoplasmic sperm injection/in vitro fertilization treatment may allow up to one in four men with Klinefelter syndrome to father children.