Most millipedes reproduce sexually via internal fertilization. The precise method differs between species and may or may not involve courtship behavior or the use of pheromones.

In millipedes, egg and sperm cells originate in gonopores in the animals' third body segments. Male millipedes have a secondary sexual organ, the gonopod, on the seventh body segment and must transfer sperm from the gonopores to the gonopod prior to copulation.

Male bristly millipedes construct a sort of web on which to deposit sperm. The female picks up the sperm and fertilizes herself. In species with courtship behavior, the male walks on the back of the female to stimulate interest. If the female is receptive, she raises her head region, allowing the male to entwine himself around her. The male deposits sperm into the female reproductive structure via a small packet, or spermatophore. In species that utilize pheromones, the chemicals are likely only effective for short-distance communication. The female lays her eggs in dirt or leaves and, in some species, guards them until they hatch.

Two species of bristly millipede are capable of parthenogenic reproduction. Males are rare or absent in those species, and reproduction is asexual, resulting in young that are essentially clones of the mother.