Protozoa (in Greek protos = first and zoon = animal) are single-celled creatures with nuclei that show some characteristics usually associated with animals, most notably mobility and heterotrophy. They belong among the eukaryotes, and are often grouped in the kingdom Protista together with the plant-like algae. In some newer schemes, however, most algae are classified in the kingdoms Plantae and Chromista, and in such cases the remaining forms may be classified as a kingdom Protozoa.

Protozoa have traditionally been divided on the basis of locomotion, although this is no longer believed to represent genuine relationships:

• Flagellates
• Amoeboids
• Sporozoans: Apicomplexa, Myxozoa, Microsporidia
• Ciliates

Most protozoans are too small to be seen with the naked eye - most are around 0.01-0.05 mm, although forms up to 0.5 mm are still fairly common - but can easily be found under a microscope. Protozoa are ubiquitous throughout aqueous environments and the soil, and play an important role in their ecology. A few are also important parasites.