The domain Eukarya encompasses a breathtaking range of size and form, from giant whales and sequoias to microbial fungi, algae, and protists. Some eukaryotic microbes are single cells as small as the smallest bacteria. Fungi include multicellular mushrooms, as well as unicellular yeasts and filamentous Penicillium and Neurospora. Algae conduct photosynthesis using chloroplasts and include broad sheets of kelp as well as unicellular phytoplankton. Protists include amebas, photosynthetic zigzag-swimming euglenas, and stalked vorticellae whose rings of cilia draw prey toward the mouth. Most protists are free-living, but some are deadly parasites, such as the trypanosome of sleeping sickness, and Plasmodium of malaria. Microbial eukaryotes serve many diverse roles in ecosystems and recent genetic surveys of environmental communities suggest the existence of twice as many protist clades in nature as we have studied to date. Many of these unknown clades may fill crucial niches in the ecosystems on which human existence depends.