The dorsal region of the porcupine is covered with thick, sharp, barbed quills, which are distributed among stiff guard hairs and wooly underfur. The number of quills on each porcupine may exceed 30,000. The quills are used for defense purposes. Each one of the modified hairs is tipped with microscopic barbs that cause the quill to be continually driven into the muscle of predators. There are no quills located on the underparts of the common porcupine.
Quills have yellowish to white bases and are dark at the tips.
Common porcupines have robust bodies with small heads, moderately small ears, short legs, and a short thick tail. The animal's feet are heavy with naked soles. The front feet have four toes while the hind feet have five toes. Each toe has a strong curved claw. The common porcupine is entirely vegetarian.