Adult isopods grow to about 1.5 cm (3/8 inch) in length. They are brownish to grayish with 7 pairs of legs and many rounded shield-like plates.

Like their aquatic cousins, terrestrial isopods breathe with gills. They are usually found under stones and moist leaf litter.

Pillbugs, including Armadillidium vulgare, can roll into a tight ball when disturbed, giving them and other pillbugs the common name of “roly-poly”.

Sowbugs possess a pair of tail-like structures on the back of the body, while pillbugs do not.