Lichens are symbiotic organisms made up of multiple species: a fungus, one or more photobionts (an alga and/or a cyanobacteria) and sometimes a yeast. They are regularly grouped by their external appearance – a characteristic known as their growth form. This form, which is based on the appearance of vegetative part of the lichen (its thallus), varies depending on the species and the environmental conditions it faces. Those who study lichens (lichenologists) have described a dozen of these forms: areolate, byssoid, calicioid, cladoniform, crustose, filamentous, foliose, fruticose, gelatinous, leprose, placodioid and squamulose. Traditionally, crustose (flat), foliose (leafy) and fruticose (shrubby) are considered to be the three main forms. In addition to these more formalised, traditional growth types, there are a handful of informal types named for their resemblance to the lichens of specific genera. These include alectorioid, catapyrenioid, cetrarioid, hypogymnioid, parmelioid and usneoid.

Lichens are composite organisms made up of multiple species: a fungal partner, one or more photosynthetic partners (also known as photobionts), and sometimes a yeast. It is a symbiotic relationship, to which each partner contributes. In most cases, the fungal partner provides the structure in which the various partners live; this structure helps to protect the photobiont from environmental pressures. The photosynthetic partner(s) provide the nutrients which the various partners need to survive. The yeast (where present) appears to help ward off microbes and potential predators through the production of various chemicals. Thallus types have evolved to provide the lichen's photobiont with optimal levels of light, water, and carbon dioxide, with different environmental conditions favouring different forms.

With the exception of calicioid lichens, lichen growth forms are based on the appearance of the thallus, which is the vegetative (non-reproductive) part of the lichen. In most species, this form is determined by the lichen's fungal partner, though in a small number, it is instead the alga or cyanobacteria (the lichen's photosynthetic partner) that determines the organism's overall shape. Growth form groupings are not always consistent with lichen taxonomy; lichens with similar growth forms are not necessarily related, and some of those which are related do not have similar growth forms. Not every species can be easily categorized. Some show characteristics of two growth forms, and different authors may place such species in different groups. In general, a particular species shows same overall growth form wherever it is found, but this is not always the case. Traditionally, crustose, foliose and fruticose are considered to be the three main forms.

An areolate lichen is the most common form of crustose lichen. As with all crustose lichens, it has a paint-like appearance, and is inseparable from the substrate on which it grows. However, its thallus is broken into regular polygonal sections, which can look a bit like cracked mud, flaking paint or little islands. These sections, known as areolae, are surrounded by a thin layer of fungal hyphae called a hypothallus. This layer, which is usually dark, generally grows faster than the thallus which rides above it. This growth form is an adaptation which allows the lichen to cope with alternating periods of wet and dry. During wet periods, the lichen can absorb water, its tissues can swell, and the cracks close. The term "areolate" is derived from the Latin word areolatus, meaning "with areolae" (the plural of a diminutive form of area, meaning "halo" or "open space") combined with the Latin suffix -atus, meaning "provided with" or "likeness".

A byssoid lichen has a wispy, cottony or teased wool appearance due to the loosely woven hyphae in its thallus. It has no outer cortex. Lichens with this growth type can be split into two types. In one type, the thallus is dominated by fungal hyphae, with a photobiont – typically a coccoid green alga – sprinkled throughout. In the other, the thallus is dominated by photobiont filaments which have a thin fungal coating. Byssoid lichens are not particularly common, but they occur across a range of orders and families. Though they are found in a variety of habitats, they appear to be most common in rainforests. Byssoid lichens typically grow in areas of high humidity, on surfaces where they have no direct contact with rainfall or running water – areas such as cactus spines in fog oases or the underside of branches in rainforests. Their loose thallus structure and lack of a cortex may allow them to absorb water vapor directly from the air. Some lichenologists consider byssoid lichens to be a specialised type of fruticose lichen. The term "byssoid" is derived from the Ancient Greek: βύσσος : býssos, a word for "linen cloth of very fine threads" (via the Latin byssus) in combination with the Latin suffix -aceus (via the English -aceous), meaning "of or pertaining to" or "with the nature of".

Unlike the other growth forms detailed here, a calicioid lichen is distinguished by its fruiting bodies rather than its thallus. Members of the order Caliciales (which gives the form its name), they are commonly known as "stubble lichens" or "pin lichens". In these lichens, mature spores build up in a thick layer on the surface of the fruiting bodies. This layer, called a mazaedium, is typically brown or black, and spores are dispersed passively from it. Most calicioid lichens are crustose with tiny stalked fruiting bodies. However, because the fundamental characteristic of a calicioid lichen is the presence of a mazaedium rather than a stalked fruiting body, a handful of fruticose lichens also fall into this category. Calicioid lichens are generally restricted to old-growth forest, and can be used as indicators of the age and quality of such ecosystems.

A cladoniform lichen is one with a dimorphic thallus. The form is named for the genus Cladonia, as most lichens in this genus show a combination of two growth types: squamulose and fruticose. The primary thallus is composed of small, overlapping scales, while the secondary thallus (which supports the lichen's fruiting structures) is fruticose in appearance. These secondary thalli, which are known as podetia, can be branched, spike-like or cup-shaped. Lichens with this growth form are found in the families Cladoniaceae and Baeomycetaceae.

A crustose lichen, as its name suggests, is crust-like and two-dimensional, closely and completely bound at nearly all points to the substrate on which it grows. It typically cannot be removed from the substrate without at least partial destruction of one or the other. Some crustose lichens are thick and lumpy, others thin and smooth, and some are almost completely submerged into the substrate with only apothecia emerging to the surface. Crustose lichens lack a lower cortex, though most have an upper cortex. The photobiont layer lies just below the upper cortex. Many crustose lichens have a ring of unlichenised fungal hyphae at their edges. This fringe, known as a prothallus, may be black, white or the same colour as the rest of the thallus. The term "crustose" derives from the Latin crustosus, meaning "crusted".