Archaebacteria are a type of prokaryote, that is, a unicellular organism without a cell nucleus. They make up the kingdom Archae, one of the main kingdoms of life. Archaebacteria are difficult to classify because they have similarities to both normal bacteria and the larger eukaryotes. In structure, they are like unicellular prokaryotes, but the genetic transcription and translation underlying their creation is similar to that of the more complex eukaryotes.

Able to live in a variety of environments, archaebacteria are known as extremophiles. Certain species are able to live in temperatures above boiling point at 100° Celsius or 212° Fahrenheit. Archaebacteria can also thrive in very saline, acidic, or alkaline aquatic environments. They employ a variety of chemical tricks to accomplish this, with one species, halobacteria, able to convert light into adenosine triphosphate (ATP) or cell energy, using a non-photosynthetic process. Halobacteria live in waters almost completely saturated with salt, and unlike photosynthetic plants, are incapable of extracting carbon from atmospheric carbon dioxide.

Archaebacteria have a size between 1/10th of a micrometer to over 15 micrometers. (A human hair is about 100 micrometers in width.) Some possess flagella, but these are substantially different in structure than the flagella bacteria have. In 1999, Pyrococcus abyssi, one of the toughest archaebacteria on Earth, had its genome sequenced. Further study of its resilience to extreme temperatures is expected to have applications in the biotechnology industry. Archaebacteria are non-pathogenic, living in and around other organisms but not infecting them. Some are able to withstand pressures of above 200 atmospheres, allowing them to thrive deep within the Earth.

Archaebacteria were not recognized as a distinct form of life from bacteria until 1977, when Carl Woese and George Fox determined this through RNA studies. However, the kingdom Archae has a close relationship to the kingdom Eukarya, the two sharing many genetic trees and common traits. One of the first places Archae were discovered was at the boiling hot springs in Yellowstone National Park.