Amphipoda are malocostracan crustaceans. They have 7 free thoracic and 6 abdominal segments, and no carapace. The antennules almost always are biramous, although the outer ramus (accessory flagellum) may be rather small. The antennal outer branch is scale-like. The first two pairs of thoracic appendages (gnathopods) are uniramous, sometimes are enlarged, and often are subchelate or variously modified. The 3rd to 7th pairs of thoracic appendages are uniramous and leg-like. There may be gills on the sternum or coxae of some or all thoracic segments. The first 2-3 abdominal segments carry long, biramous pleopods, and segments 4 and 5 always carry paired, backward-facing uropods. A third pair of uropods, on abdominal segment 6, are vestigial in one family but well-developed in all others. They may be equal in size to uropods 1 and 2 or else much longer. When the third uropods are short the body terminates with the telson. When the third uropods are long the telson can be recognised as a small projection (often cleft) apparently from the dorsal surface of abdominal segment 6.

Amphipoda do not differ greatly in overall structure from Isopoda. The main taxonomic features are that Isopoda have only a single pair of uropods, may have the pleopods modified as gills (or in terrestrial species as pseudo-lungs), and have no gills on the thorax. Amphipoda have 2-3 pairs of uropods, the pleopods are unmodified, and gills (if present) are on the thoracic sterna or coxae.

Amphipoda nearly always are flattened laterally while some Isopoda (e. g., pill-bugs) are flattened dorso-ventrally. However, this is not of much use for identifying aquatic specimens to order because a majority of aquatic isopods are, like amphipods, somewhat flattened laterally.

A large number of amphipod families occur in Australian inshore and estuarine waters. The extent to which some of these may be found in non-marine waters is unclear. We follow Bradbury and Williams (1999) in accepting that nine families are genuinely aquatic in Australia. To these we have added the one family of terrestrial Amphipoda, Talitridae. These live in damp situations and often occur as margin-dwellers or as accidental inclusions in aquatic samples.

Aquatic amphipods occur primarily in the southern half of Australia.