Almost 70% of the sharks living today have developed a method to care for their brood inside the female's body. In many aspects it resembles the reproductive method of mammals. The eggs are fertilized in the upper part of the fallopian tube and are encased in only a very thin membrane. The females then retain the developing eggs in the last extended section of the fallopian tube which is thus termed the womb or uterus. Protected by the mother's body, the pups develop and are born as self-sufficient sharks following a gestation period of 6 to 22 months. The size of the pups at birth is usually between 45 and 60 cm.

Approximately 30% of all sharks lay eggs. These eggs are round to oval and are encased in a hornlike egg capsule. The protective egg capsules are composed of two to three cases and often have interesting shapes, e.g. cordlike growths which serve to anchor the eggs on water plants or other objects. This prevents the eggs from being swept away.
Normally the eggs are quite large, measuring 10 to 25 cm in length. They also have a large yolk which nurtures the embryos sufficiently with nutrients. By laying their young in well protected, stable egg capsules with sufficient nutrients, the female sharks shorten the time needed to care for their young.