Parasitic arthropods, particularly crustaceans, play significant roles in aquatic ecosystems, utilising various hosts for their life cycles. This chapter delves into the biology and life cycles of selected parasitic arthropods in aquatic ecosystems, especially highlighting crustaceans that exhibit significant diversity, often involving complex life cycles with multiple hosts. Ten major crustacean groups, in six classes, are discussed in the chapter. Life cycle notes on certain parasitic copepods from Cyclopoida, Harpacticoida, Monstrilloida and Siphonostomatoida are provided. Within the class Ichthyostraca, the Branchiura (or ectoparasitic fish lice) and the Pentastomatida (worm-like crustaceans) are highlighted. The third class, Malacostraca, includes amphipods, such as whale lice and jelly parasites, as well as the parasitic isopods from Bopyridae, Cymothoidae and Gnathiidae. The parasitic ostracods and the tiny Tantulocarida are mentioned, followed by the final aquatic crustacean class, Thecostraca. This last class includes the Ascothoracida (endoparasites within echinoderms and cnidarians), the Cirripedia (comprising the Acrothoracica, Rhizocephala and Thoracica) and what is known of the enigmatic Facetotecta. Additionally, the terrestrial arthropods within Arachnida and Hexapoda (Insecta) parasitising aquatic hosts are discussed. Throughout this chapter, the ecological implications of parasitic crustaceans are emphasised, particularly their roles in host population dynamics and potential co-evolutionary interactions. Understanding these life cycles provides valuable insights into the ecological impact of parasitic arthropods on their hosts and the broader aquatic environment.