© 2001 by CRC Press LLC. Meroplankton is comprised of organisms that spend some component of their life history in the plankton, usually the eggs and larvae of benthic or nektonic adults. There are a few examples of adult benthic marine invertebrates that spend brief periods of time in the plankton. These include the adult reproductive phase (epitoke) of some marine polychaetes, whose benthic life history is interrupted at reproductive maturity by a dramatic ontogeny of swimming appendages followed by swimming behaviors that result in the swarming and bursting of a pelagic reproductive phase. Some deep sea holothuroids (sea cucumbers) spend some period of their adult life swimming in the plankton. Moreover, there are a number of demersal marine invertebrates such as copepods and amphipods that migrate up into the water column at night. Nonetheless, the vast majority of meroplankton in the world’s oceans are comprised of the propagules of algae or the eggs and larvae of benthic invertebrates and fish. Little is known of the chemical defenses of the propagules of algae or the eggs and larvae of fish. Therefore, the present review and discussion of the chemical ecology of meroplankton will focus primarily on feeding-deterrent properties of marine invertebrate eggs and larvae. While all marine invertebrate groups have the potential of producing defensive chemistry in their larval offspring, studies to date have focused specifically on the eggs and larvae of sponges, cnidarians, molluscs, echinoderms, and ascidians, groups of organisms that are well known to possess chemical defenses in their adult stages.