What could cause bald spots in hair?

Alopecia areata. Alopecia aerata can cause smooth bald spots on the head. This is an inflammation which cause the hair to stop growing underneath the skin. This can occur after an emotional upset and is treatable with small injections of dilute cortisone to stimulate hair growth.
Multiple causes. Autoimmune disease, hair-pulling, wear (constant rubbing of scalp on one spot), congenital. How old is the patient?