Over the last years, the rapid advance of discoveries in the field of molecular genetics has provided physicians and scientists with new insights into etiology and pathogenesis of several monogenetic and polygenetic human diseases. Along with remarkable progress regarding genodermatoses in general, different molecular mechanisms involved in hair growth have been elucidated and mutations causing several genetic hair disorders have been identified. We provide an overview of the molecular genetic basis and the clinical hallmarks of some diseases associated with hypertrichosis and hypotrichosis.