Although the majority of lichens are terrestrial, some grow in areas subject to salt spray and are a conspicuous feature of the upper reaches of many rocky shores. Some withstand daily submersion by the tide and are found in the eulittoral zone, where they compete for space with other organisms, for example, barnacles and seaweeds. Lichens are complex plants composed of fungal and algal components existing in what is believed to be a mutually beneficial association known as symbiosis in which photosynthesis of the algal cells provides the fungus with organic nutrients, while the fungus provides the alga with support and protection. The exact nature of the relationship is still debated. Each species of lichen has its own species of fungus but the algal component is not specific to a particular fungus, and it is believed that the algae can exist independently, unlike the fungus which survives only in the lichen association.

The main part of the lichen, known as the thallus, is typically seen in one of three forms; crustose, foliose and fruticose. The flattened, crust-like thallus of crustose lichens is firmly attached to the substratum and very difficult to remove without damage to the lichen. Foliose lichens have a horizontal, leaf-like thallus usually loosely attached to the substratum and often divided into lobes. In the fruticose lichens the thallus stands upright or hangs downwards from the base.