Kallmann syndrome is characterized by hypogonadotropic hypogonadism and anosmia/hyposmia. The hypogonadotropic hypogonadism is due to deficiency of gonadotropin-releasing hormone, caused by a defect in the migration of neurons synthesizing gonadotropin-releasing hormone, and anosmia/hyposmia is related to the absence or hypoplasia of the olfactory bulb and tracts. Some patients may have other associated abnormalities such as renal agenesis, cleft palate, dental agenesis, synkinesis, shortening of metacarpal, sensory neural hearing loss and seizures. The aim of this paper is to present an updated review of the clinical and molecular basis, highlighting the relevance of knowledge of phenotypic variants in Kallmann syndrome.