The Kingdom Protista or Protoctista is one of the commonly recognized biological kingdoms, including all the eukaryotes except for the plants, fungi, animals, and sometimes other groups which are treated in separate kingdoms. A few forms are multicellular, for instance the brown and red algae. The vast majority, though, comprise the single-celled organisms, and are typically only 0.01-0.5 mm in size, usually too small to be seen without a microscope. Protists are ubiquitous throughout aqueous environments and the soil, commonly surviving dry periods in the form of cysts; a few are important parasites.

Traditionally protists have been divided into:

• Plant-like forms that have chloroplasts, the algae:
  • Green algae,
  • Red algae,
  • Heterokont algae,
  • Plus various other groups;
• Fungus-like forms:
  • Slime moulds,
  • Water moulds
• Animal-like forms, the protozoa, generally divided on the basis of morphology and locomotion into:
  • Flagellates (e.g., Euglena),
  • Amoeboids (e.g., Amoeba),
  • Apicomplexa (e.g., Plasmodium),
  • Ciliates (e.g., Paramecium)

In early classifications the protozoa were considered a phylum of animals, and the algae and slime moulds were placed among several divisions of plants. Many forms were classified under both kingdoms and researched by zoologists and botanists alike. Eventually the kingdom Protista was created to house these forms, with the classes of protozoa (corresponding roughly to the above) being promoted to phyla. Except for the ciliates and water moulds, however, all the above groups are polyphyletic and frequently overlapping. Further, the protists themselves are understood to be paraphyletic to the other eukaryotic kingdoms.

More recently attempts have been made to divide protists into more genuine groups on the basis of ultrastructural, chemical, and genetic features. In newer classification systems these are often treated as separate kingdoms. However, there are still many different lines of protists whose relationships are not understood. Many scientists now consider the various protist clades as direct subgroups of the eukaryotes, with the admission that we do not yet know enough about them to properly arrange them into a hierarchy. A few major groups are listed at right; others are described under the pages linked to above.