A litter is a group of newly born, young animals from the same mother and usually from one set of parents. The word is most often used for a group of baby mammals, but can refer to the young of any animals that give birth to multiple young. In comparison, a group of eggs and the offspring that hatch from them are frequently called a clutch, whilst young birds are often called a brood.

Animals frequently display grouping behavior in herds, swarms, flocks, or colonies, and these multiple births derive similar advantages. A litter offers some protection from predation, not particularly to the individual young but to the parents' investment in breeding. With multiple young, predators could eat several and others could still survive to reach maturity, but with only one offspring, its loss could mean a wasted breeding season. The other significant advantage is the chance for the healthiest young animals to be favored from a group. Rather than it being a conscious decision on the part of the parents, the fittest and strongest baby competes most successfully for food and space, leaving the weakest young, or runts, to die through lack of care.

In the wild, only a small percentage, if any, of the litter may survive to maturity, whereas domesticated animals and those in captivity with human care may see the survival of the whole litter. Animals that do not produce multiple births may mimic the advantages of multiple births by breeding in colonies where breeding is timed to coincide with the birth of many other young animals.