A unicellular organism is any life form that consists of just a single cell. Most of life is unicellular, with bacteria serving as the majority. The main groups of unicellular life are bacteria, archaea (both prokaryotes), and the eukaryota (eukaryotes). The differences between the prokaryota and eukaryota are significant: eukaryotes possess a nucleus, while prokaryotes lack it, and eukaryotes possess a range of subcellular organs called organelles, while prokaryotes are very minimal.

You can observe the larger unicellular organisms, such as amoebae, by using the higher settings on a light microscope. Bacteria just appear as dots. To gather unicellular organisms for observation, one can place a cover slip on the surface of pond water, and leave it overnight. By the next morning, numerous unicellular organisms will have grown entire colonies on the bottom of the slip. Unicellular organisms replicate fast: colonies can double their size in between 30 minutes and a few hours.

Unicellular organisms as diverse as they are ubiquitous. The oldest forms of life, unicellular organisms existed 3.8 billion years ago, if not longer. They pursue a variety of strategies for survival: photosynthesis (cyanobacteria), chemotrophy (many archaea), and heterotrophy (amoeba). Some unicellular organisms have flagella, little tails they use for locomotion, or lobopods, extensions of the cellular skeleton (cytoskeleton), which appear as bloblike arms. The flagella of our unicellular ancestors is retained all the way up into the animals, where it makes an appearance as flagellated sperm.

Of all the six eukaryote supergroups, four are exclusively composed of unicellular organisms. Only the opisthokonts, consisting of animals, fungi, and close relatives, and the archaeplastids, consisting of both unicellular and multicellular plants, are exceptions. Unicellular organisms vary in size, with the smallest bacteria measuring only a third of a micron (300 nanometers) across, ranging up to the titantic plasmodial slime molds, which can grow to 20 cm (8 in) across. The largest unicellular organisms may have millions of nuclei scattered throughout the cellular envelope. To observe some of the smallest unicellular organisms requires an expensive electron microscope, why the very largest can be seen with the naked eye.