Klinefelter syndrome (KS) is the most frequent chromosome disorder in men affecting 1 in 600 live male births. It is also the commonest cause of male infertility. The predominant subclass is classic 47,XXY accounting for 80-90% of cases with the remainder due to higher grade aneuploidies, mosaicisms or structurally abnormal X chromosomes. Whilst the clinical presentation is highly variable, the disorder is consistently characterised by small testicular volume and hypergonadotrophic hypogonadism which may go unnoticed until adulthood. Morbidity and mortality are increased in those with KS with a greater burden of cardiovascular, metabolic and bone-related disease, as well as neurocognitive and psychosocial issues. Timing of pubertal onset is normal, but germ cell depletion accelerates during puberty together with progressive fibrosis and hyalinisation of seminiferous tubules and Leydig cell hyperplasia. Ejaculated spermatozoa is exceptional in adults, yet foci of spermatogenesis remain and sperm retrieval rates up to 75% with micro-TESE are reported. This presentation will provide an overview of the diagnosis and clinical presentation of KS, testis development and fertility potential, fertility preservation options, and non-endocrine aspects of KS.