The Kingdom Protista includes several phylogenetically distinct groups, associated together because they are all simple eukaryotes. Like more complex eukaryotes, protists have membrane-bound organelles and a defined nucleus. However, they are not organized into complex tissues, as are higher plants and animals. For ease, we will divided them into the larger categories of algae, protists and slime molds.

Algae are plant-like photoautotrophs and have cell walls. Algae are difficult to define, and several different classification systems exist. In this SparkNote, we will treat all eukaryotic photoautotrophic microorganisms as algae. This includes five groups: green algae, red algae, brown and yellow-brown algae, dinoflagellates, and euglenoids. Algae are extremely important as producers in the world's oceans and lakes.

Protozoa are animal-like. Protozoa lack cell walls and are eukaryotic heterotrophic microorganisms. They include ciliates, kinetoplastids, and sarcodina. Many protozoa live as parasites within other animals.

Slime molds are unusual microorganisms that resemble protozoa at some phases of their life cycles and fungi at other phases. The two groups of slime molds, cellular and acellular, are not closely related. Slime molds live in damp soils and decaying plant material.