A lichen is a plant formed from the symbiotic association of certain fungi and (usually) green algae. Lichens with associated cyanobacteria are also known. Lichens occupy many different habitats, often in extreme environments.

Three main types of lichens exist in Antarctica. Crustose lichens, which form a thin crust on the surface of the substrate they grow on; foliose lichens, which form leaf like lobes; and fruticose lichens which have a shrubby growth habit.

Lichens have very slow growth rates. In the most favourable of conditions in the Maritime Antarctic, growth rates reach 1 cm or more per 100 years. In the harsher environment of the Continental Antarctic, growth is much slower, and may be as little as 1cm per 1000 years for Buellia frigida in the McMurdo Dry Valleys region.

Lichens can be found growing in most areas of the Antarctic capable of supporting plant life. Currently four general distributional patterns of lichens are known. These are: species confined to the Maritime Antarctic; those found in the Peninsula and extending to the Lesser Antarctic; those with a Circum Antarctic distribution; and those with very disrupted or Disjunct distribution patterns.

The Maritime Antarctic lichens are restricted to the northern Peninsula and nearby islands. Many of the lichens found in Antarctica are restricted to this area. A number of the lichen species found here are also found in the subantarctic islands and the colder parts of the southern continents, and may represent a southern extension of these populations. This area has the greatest species diversity in Antarctica.

Lichens have been collected from as far south as 86°30'.

Lichens have a number of adaptations that enable them to survive in Antarctica. They are able to exhibit net photosynthesis while frozen at temperatures as low as −20°C. They can absorb water from a saturated atmosphere when covered by snow. Additionally, snow cover affords protection from the elements and most growth appears to occur when they are buried beneath at least a thin protective layer of snow. They can survive long unfavourable periods of drought in a dry and inactive state. In continental Antarctica, many lichens are able to absorb water vapour from snow and ice.