Reproduction can be highly varied due to multiple different organisms in this phylum. Reproduction first occurs between two opposite sex. A female will lay the egg and the male will fertilize with the sperm. But some mollusks are hermaphrodites so they have both the male and female reproductive system. Sea slugs and oysters can repeatedly change their sex to protect their child.

Fertilization can occur externally or internally. External reproduction occur most between aquatic mollusks. The male and female mollusks will eject their egg or sperm into the water and fertilization and development begins. Within 2-3 weeks the egg will hatch into a larva known as trocophore. Trocophores are spherical pear shaped and surrounded by cilia. They use the cilia to sweep in floating food particles. After the trocophore stage, they develop into the veliger larva and a velum forms. The velum is used for swimming, feeding, and gas exchange. In this stage, they also start to develop a foot. This is also the stage where they develop their adult features, and are known as miniature adults.

Some other mollusks reproduce internally, such as terrestrial snails. Fertilization takes place in the female when the male's sperm has successfully fertilized the egg. For some species, when the egg is fertilized the female will lay the egg into the water. A larvae or a miniature adult is formed when hatched. These larvae will experience the same stages as the larvae developed through external reproduction, trocophore and veliger. In other mollusks, the fertilized eggs remain inside the female where they are fully developed and then born.