The word micro-organism originated from the Greek word, mikrós, meaning "small" and organismós, meaning organism. Microorganisms or microbes are organisms that are microscopic in size and these are living organisms which are usually too small to be seen by an unaided human eye. There are hundreds of micro-organisms in a single drop of water, a pinch of soil and the air that is all around us.

The study of micro-organisms is called microbiology, a subject which began with Anton van Leeuwenhoek's discovery of micro-organisms in 1675, using a microscope of his own design. Robert Hooke was the first person to use a microscope to observe living things; his 1665 book Micrographia contains descriptions of plant cells.

Micro-organisms are very diverse; they include bacteria, fungi, archaea, and protists; microscopic plants (called green algae); and animals such as plankton, the planarian and the amoeba. Some microbiologists also include viruses, but others consider these as non-living. Most micro-organisms are unicellular (single-celled), but some multicellular organisms are also microscopic, while some unicellular protists and bacteria, like Thiomargarita namibiensis, are macroscopic and are visible to the naked eye.

Micro-organisms live in all parts of the biosphere where there is liquid water, including soil, hot springs, on the ocean floor, high in the atmosphere and deep inside rocks within the Earth's crust. They are very important for nutrient recycling in ecosystems as they act as decomposers. As some micro-organisms can fix nitrogen, they are a vital part of the nitrogen cycle.

Microbes are very useful and are exploited by people in biotechnology, both in traditional food and beverage preparation, and in modern technologies based on genetic engineering. However, pathogenic microbes are harmful, since they invade and grow within other organisms, causing diseases that kill millions of people, other animals, and plants.

Single-celled micro-organisms were the first forms of life to develop on Earth, approximately 3–4 billion years ago and for most of the history of life on Earth, micro-organisms were the only forms of life. Bacteria, algae and fungi have been identified in amber that is 220 million years old, which shows that the morphology of microorganisms has changed little since the triassic period.

Most microbes can reproduce quickly and microbes such as bacteria can also freely exchange genes by conjugation, transformation and transduction between widely-divergent species. This transfer of genes, coupled with a high mutation rate allows micro-organisms to swiftly evolve to survive in new environments and respond to environmental stresses. This rapid evolution is important in medicine, as it has led to the recent development of 'super-bugs' — pathogenic bacteria that are resistant to modern antibiotics.

Micro-organisms are found in almost every environment in nature including hostile environments such as the poles, deserts, geysers, rocks, and the deep sea. Some microorganisms known as extremophiles have adapted to the extreme conditions. Extremophiles have been isolated from rocks 7 kilometres below the Earth's surface, and it assumed that the amount of living organisms below the Earth's surface may be comparable with the amount of life on or above the surface. Extremophiles have been known to survive for an extended time in a vacuum, and can be highly resistant to radiation, which may even allow them to survive in space. Many types of microorganisms have intimate symbiotic relationships with other larger organisms; some of which are mutually beneficial, while others can be damaging to the host organism. When micro-organisms cause disease in a host they are known as pathogens.

Micro-organisms are used in brewing, winemaking, baking, pickling and other food-making processes. Specially-cultured microbes are used in the biological treatment of sewage and industrial waste effluent. Microbes are used in fermentation to produce ethanol, and in biogas reactors to produce methane. Research is being conducted to utilise bacteria to convert various forms of agricultural and urban waste into usable fuels. Microbes are also essential tools in biotechnology, biochemistry, genetics, and molecular biology. Microbes are being used as a solution for pollution as in the use of Super Bug for controlling oil-spills.

Micro-organisms are essential to human beings and the environment, as they contribute to the Earth's element cycles such as the carbon cycle and nitrogen cycle, as well as in recycling other organisms' dead remains and waste products through decomposition. Any improper balance of these microbes may prove to be disastrous. They are absolutely crucial to the health and well being of the Earth and without these microorganisms; the Earth would be a massive dump of waste.