A dermatologist is a doctor who specializes in conditions involving the skin, hair, and nails. A dermatologist can identify and treat more than 3,000 conditions. These conditions include eczema, psoriasis, and skin cancer, among many others. A dermatologist is a doctor that specializes in treating skin, hair, nail, and mucous membrane disorders and diseases. They can also address cosmetic issues, helping to revitalize the appearance of the skin, hair, and nails.