A board-certified dermatologist is a medical doctor who has spent four years after medical school becoming an expert in the care of the skin and the treatment of skin diseases. Dermatologists treat a range of conditions including acne, rosacea, psoriasis, autoimmune diseases that affect the skin, eczema, and many many more skin disorders. Dermatologists also check your skin annually to ensure that no moles have changed and no skin cancers have arisen.