Scuds or sideswimmers are a type of crustacean called amphipod (meaning "both-foot") because the front thoracic legs are adapted for walking and the rear ones for swimming. They have a laterally flattened body, well-developed eyes, and two pairs of antennae. They both walk on the bottom and swim freely, often rapidly on their sides.

Scuds live close to the bottom, avoiding bright light. Here they can scavenge on microscopic plants or animals and decomposing material, while being protected from predators. The female keeps the fertilized eggs in a brood pouch, or marsupium, where the eggs hatch after 1 to 3 weeks. Young stay in the pouch until their mother molts again in a week or so.

The name scud comes from the Norwegian word "skudda" meaning "push" and refers to the way they move swiftly.