Brachycephaly (derived from the Ancient Greek βραχύς, 'short' and κεφαλή, 'head') is the shape of a skull shorter than average in its species.

It is perceived as a cosmetically desirable trait in some domesticated dog and cat breeds, notably the pug and Persian, and can be normal or abnormal in other animal species.

The usefulness of the cephalic index was questioned by Giuseppe Sergi, who argued that cranial morphology provided a better means to model racial ancestry.

[2] When pathological, it is known as flat head syndrome, and it results from premature fusion of the coronal sutures, or from external deformation.

[6] In pets, brachycephaly describes a type of skull with a high cephalic index, such as in snub-nosed breeds of dog such as pugs, Shih Tzus, and bulldogs or cats such as the Persian, Exotic and Himalayan.

[8] Brachycephalic cat breeds such as the Burmese, Himalayan and Persian normally have mandibular mesioclusion — a condition involving the incisors not being aligned correctly.

All parents of infants treated with helmets confirmed negative side effects including skin irritation and sweating.