Generalized hair loss is a problem associated with the loss of the entire coat and is one of the main causes of veterinary visits to animals. In most cases, it is an acquired condition, although there are also congenital forms of hair loss, caused by abnormal development of hair follicles. This article discusses both types of causes of hair loss in dogs. Congenital hair loss can occur in the first weeks of puppies' life, but it does not progress with age. Although not always, most often congenital hair loss has a genetic basis. Dysplasia with bleaching of hair color and follicular dysplasia of black hair are examples of congenital skin neuroectodermal disorders. Acquired hair loss is usually caused by hypothyroidism, hyperadrenocorticism and hyperestrogenism, and occurs in dogs at a later age associated with hormonal disorders.