The order Harpacticoida is one of the ten orders of the sub-class Copepoda which, along with six other sub-classes, make up the class Maxillopoda, one of the classes of the lower Crustacea. Most species are free-swimming in marine or brackish water, although a few are ectoparasitic or commensal. The marine harpacticoids are primarily bottom-living copepods, although a few are exclusively planktonic, and are most abundant in soft sediments and on macro-algae.
There are approximately 800 species belonging to around 190 genera of marine harpacticoids occurring in the waters around the British Isles. This Synopsis provides keys to generic level for the sub-order Polyarthra and 14 families of the sub-order Oligoarthra.