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Monstrilloids are one of the most intriguing groups of copepods. Their complex life cycle represents the successful evolutionary outcome of dealing with three distinct kinds of habitat, viz., planktonic, benthic, and endoparasitic, each of which presents particular challenges that have been overcome by monstrilloids. These copepods combine a unique set of strategies and adaptations to complete their life cycle. The non-feeding planktonic adult phase lacks mouthparts and their antennules are fixed, thus limiting their swimming abilities but they compensate for this handicap by having powerful swimming legs and probably generate a very distinct hydrographic signal that may be useful in avoiding predators and allowing sexual recognition between adult males and females. Parasitizing exclusively on abundant, gregarious sessile or sedentary benthic organisms represents an advantage in that potential hosts can be found without the need for long-distance dispersal. The endoparasitic stages of monstrilloids are unique; after infection by an early planktonic nauplius, successive nauplioid stages feed on their own vitellum while developing feeding tubes to absorb nutrients from their hosts. They grow within the host’s body as successive copepodite stages that are contained in a protective sheath. Preadult individuals exit through the host body wall causing significant host damage or death, behaving in these instances as parasitoids. The diversity of the group appears to be underestimated, and extensive geographic areas remain completely unknown for this group of copepods. More effort will be required to advance our knowledge of monstrilloid diversity and biology that are yet to be revealed.