Protists are a polyphyletic group of diverse, mostly microscopic, unicellular organisms that encompass all species that are not animals, plants or fungi. One common characteristic of protists is that they prefer aquatic or moist habitats and here we find marine picoplankton, the smallest free-living protists, as well as the giant kelp, which is the largest. Of the many known protist species and life forms, roughly 15,000 are parasitic. The relationship of parasitic protists with their hosts ranges from harmless to causing significant diseases in humans, animals and plants. Here we focus on parasitic protists in the aquatic realm, including amoebae, ciliates, flagellates and sporozoans.