The basic unit of killer whale social structure is the matriline. This means that the most common and most consistent grouping of killer whales consists of a mother and her growing offspring.

Killer whales mature slowly and it is not unusual for young whales to stay with their mothers for more than a decade. Long after the babies stop nursing, they presumably benefit by staying with their mothers because of the protection that she can provide and because of the need to learn the hunting lifestyle by observing and gradually acquiring their mother's skills. Thus, when one encounters a group of whales in the wild, it is most likely to consist of an adult female accompanied by two or three of her offspring -- spaced out over a range of ages.

Among the "Resident" whales in the North Eastern Pacific, matrilineal cohesion is extreme. In this ecotype of orca, both male and female offspring remain with their mothers long into adulthood such that large pods form that consist of a grandmother, a number of her adult male and female offspring, and new calves that are born to her daughters.