They belong to the order Isopoda, suborder Oniscidea, with over 3000 known species.
In Britain there are 37 native or naturalized species ranging in colour and in size (3 - 30 mm) of which only five are common - Oniscus asellus (Common shiny woodlouse), Porcellio scaber (Common rough woodlouse), Philoscia muscorum (Common striped woodlouse), Trichoniscus pusillus (Common pygmy woodlouse) and Armadillidium vulgare (Common pill bug).