This is an introductory course on the diversity of eukaryotes in different aspects — from genomes to ecological interactions. Major topics include evolutionary relationships and history, cell structures and functions, and importance for various ecosystems and humanity. More emphasis is put on eukaryotes less covered in other courses — namely protists (algae and non-photosynthetic microbial eukaryotes) — which occur in oceans, soils, lakes, animal guts and many other environments.