They can be lost or lacking for a number of reasons—physical injury, burns, disease, chemotherapy, radiation, scarring caused by long-term plucking, and congenital inability to grow them.

Eyebrows and eyelashes can often be restored by (1) transplantation of hair-bearing skin from another area, or (2) flap surgery to move a flap of hair-bearing skin from the scalp to the eyebrow.  Eyebrow and eyelash restoration requires a skilled and experienced surgeon.