Microorganisms or microbes are organisms that are very small (usually less than 1 mm) so that to observe it requires a relief tool. Microorganisms are often single-celled (unicellular) although some single-celled protists are still visible to the naked eye and there are some multicellular species invisible to the naked eye. The study of microorganisms is called microbiology.

Microorganisms are usually considered to include all prokaryotes, protists and micro algae. Fungi, especially small ones and do not form hyphae, can also be considered as part of it even though many do not agree on it. Most people assume that what can be considered microorganisms are all very small organisms that can be cultured in petri dishes or incubators in the laboratory and are able to multiply in mitosis.