Fungi, protists, and algae are among the most diverse life forms on the planet. Tiny protists range from plants to animals. Algae range from single-celled plants to 6-foot-long kelp. Fungi occupy a kingdom of their own. Although their place in nature—neither truly plants or animals—may make them seem strange, they are actually quite familiar to us for their use in various foods, medicines, and biotechnologies. Indeed, our consumption of bread or reliance on penicillin would not be possible without these diverse eukaryotic organisms. This intriguing volume delves into the taxonomy, structure, and function of each group and evaluates their wide-ranging natural and societal impact.