Congenital abnormalities of the hair shaft are conditions in most cases linked to chemical, biochemical, and morphological alterations, genetically determined. These alterations may be associated with a larger array of symptoms, as mentioned above, or may occur isolated. The number of genes involved and their penetration are responsible for the mode of transmission, severity, and phenotypic expression of the disease. Furthermore, in some cases, these abnormalities can be considered true pathognomonic markers of disease. Within this disorder, two groups are classically identified: with increased fragility of the hair shaft and without increased fragility. Congenital abnormalities of the hair shaft that present an increased fragility of hair include: Trichorrhexis nodosa congenita, Trichoschisis, Trichorrhexis invaginata, Trichothiodystrophy, Trichoclasia, Pili torti, Monilethrix and Loose anagen syndrome; while the alterations that are not associated with a greater fragility of the hair are: Pili annulati, Longitudinal grooves, Uncombable hair syndrome (Pili trianguli et canaliculi), Woolly hair, Woolly hair with dominant inheritance, Familial recessive woolly hair and Woolly hair nevus.