Grey seal breeding seasons vary, but most breeding takes place between late September and early March. Males posture or fight for access to multiple females, which congregate at haul-out sites to bear and nurse their young. Gestation lasts ten months to a year, and pups are born at haul-out sites the season following mating. They wean about three weeks after birth. Adult females breed again at about the same time as their young wean.
Females usually start breeding at three to five years old.
Although males are ready to breed at four to eight years old, due to competition with older males they rarely do so before ten years old. In zoos, they can live into their forties. Grey seals congregate in large groups for breeding, pupping, and molting. Size and fat reserves play an important part in successful breeding. Male seals that can spend more time on land chasing females, and less time feeding at sea, have greater mating success.