Clinical presentation of Klinefelter’s syndrome is highly heterogeneous, explaining why the disease is often undiagnosed. Klinefelter patients have traditionally been described as tall, with gynecomastia, small and firm testicles. Beyond this classical phenotype, less distinct phenotypes have been described with less evident features. The reduction of the volume of the testes remains the peculiar clinical hallmark of Klinefelter’s syndrome. The broad spectrum of Klinefelter phenotypes depends on the genetic background (mosaicism, number of supernumerary X chromosome) as well as the degree of hypogonadism, patient’s age, and delay in the diagnosis. The relationship between clinical phenotype and pathogenesis is still not completely understood and needs to be fully elucidated, in order to facilitate the diagnosis and the clinical management of this condition. As expected, the degree of severity of the phenotype influences psychosocial issues and quality of life in these patients.