The causes of darkening on the back of the hands include long-term exposure to the sun, post-inflammatory hyperpigmentation, and seborrheic keratosis, etc. Treatment should be carried out according to different causes. 1. Long-term exposure to the sun: the back of the hand is exposed to the sun for a long time and receives a lot of ultraviolet radiation, which leads to an increase in pigment particles in the epidermal cells of the skin and then the back of the hand becomes darkened, and may also be accompanied by roughness of the back of the hand, etc. The back of the hand should be less exposed to the sun or sunscreen measures should be taken, and the back of the hand should not be over-exposed to the sun. 2. Post-inflammatory hyperpigmentation: When the skin on the back of the hand suffers from dermatitis, eczema and other inflammatory skin diseases, it is easy to leave behind hyperpigmentation after the disease has healed, i.e., the back of the hand appears to be dark. 3. Seborrheic keratosis: some older people may have age spots, which are characterized by light brown spots on the back of the hands, gathered in large quantities, and darkening of the back of the hands. When the back of the hand darkening, should pay attention to avoid long-term sun exposure, do a good job of protection, if necessary, can be coated under the guidance of the doctor to lighten the back of the hand skin pigmentation of drugs, if accompanied by physical discomfort, should be timely medical care.