Free-living copepods are small (max. 4mm) with a well-defined head and a single, central eye.

The head is fused with the anterior thoracic segments to form a cephalothorax in which the head part is called the cephalosome, the thoracic part the metasome. The abdominal segments comprise the urosome.

The head carries elongate and conspicuous antennules which are the main swimming organs. Behind these are much smaller antennae, followed by the mandibles and two pairs of maxillae (or maxillules and maxillae).

Thoracic appendages comprise the first legs or maxillipeds, the second, third, and fourth legs (or 1st., 2nd., and 3rd. swimming legs), and the fifth legs. These last are small and generally modified for mating, and are important in family and generic identification. The abdominal segments usually are without appendages, except the caudal (terminal) segment bears two rami ('tails') which may end in long setae.

All appendages except the antennules and in some genera the antennae are small and only visible in lateral or ventral view.

Copepoda are widespread, typically planktonic organisms. Eggs hatch to give larvae (nauplii) which attain mature forms through a fixed series of moults. Some species have desiccation-resistant eggs. Copepods are important items in the diet of many freshwater fish.

Three orders (or superfamilies) occur in Australia. Some species and genera of superfamilies Cyclopoida and Harpacicoida are parasitic, mainly on fish. Many groups are marine. The free-living Copepoda of inland waters have been assigned to five families:

Cyclopidae (Cyclopoida),

Centropagidae and Diaptomidae (Calanoida),