The condition of dark spots on arms may be due to seborrheic keratosis, congenital nevus, coffee spots and other factors. 1. Seborrheic keratosis: this is a relatively common benign tumor of the epidermis, which usually occurs in adults over 40 years old, and will appear in any skin of the body, with round or oval plaques, which are light brown or black in color, and is also known as age spots, which is mainly caused by aging of the skin as well as genetic susceptibility. 2. Congenital nevus: it is considered to be some kind of misshapen tumor or benign proliferation of melanocytes, it is considered to be related to congenital heredity and developmental defects, it may occur on the head and face, or on the limbs and trunk. 3. Coffee spot: most of them are hereditary diseases, which can be found from birth to childhood, mostly occurring on the face and trunk, and generally need no treatment. There may be other reasons for arm black spot, so it is necessary to cooperate with doctor to improve the examination to be clear, and should be given targeted treatment and therapy.