For people, the relevance and functionality of teeth can be easily taken for granted, but a closer examination of their considerable significance will demonstrate how they are actually very important. Among other things, teeth serve to:

- support the lips and cheeks, providing for a fuller, more aesthetically pleasing appearance

- maintain an individual's vertical dimension of occlusion

- along with the tongue and lips, allow for the proper pronunciation of various sounds

- preserve and maintain the height of the alveolar ridge

- cut, grind, and otherwise chew food

When an individual's mouth is at rest, the teeth in the opposing jaws are nearly touching; there is what is referred to as a "freeway space" of roughly 2–3 mm. However, this distance is partially maintained as a result of the teeth limiting any further closure past the point of maximum intercuspation. When there are no teeth present in the mouth, the natural vertical dimension of occlusion is lost and the mouth has a tendency to overclose. This causes the cheeks to exhibit a "sunken-in" appearance and wrinkle lines to form at the commissures. Additionally, the anterior teeth, when present, serve to properly support the lips and provide for certain aesthetic features, such as an acute nasiolabial angle.

Loss of muscle tone and skin elasticity due to old age, when most individuals begin to experience edentulism, tend to further exacerbate this condition.

The tongue, which consists of a very dynamic group of muscles, tends to fill the space it is allowed, and in the absence of teeth, will broaden out. This makes it initially difficult to fabricate both complete dentures and removable partial dentures for patients exhibiting complete and partial edentulism, respectively; however, once the space is "taken back" by the prosthetic teeth, the tongue will return to a narrower body.

Angular cheilitis is a fairly non specific term which describes the presence of an inflammatory lesion in a particular anatomic site (i.e. the corner of the mouth). As there are different possible causes and contributing factors from one person to the next, the appearance of the lesion is somewhat variable. The lesions are more commonly symmetrically present on both sides of the mouth, but sometimes only one side may be affected. In some cases, the lesion may be confined to the mucosa of the lips, and in other cases the lesion may extend past the vermilion border (the edge where the lining on the lips becomes the skin on the face) onto the facial skin. Initially, the corners of the mouth develop a gray-white thickening and adjacent erythema (redness). Later, the usual appearance is a roughly triangular area of erythema, edema (swelling) and breakdown of skin at either corner of the mouth. The mucosa of the lip may become fissured (cracked), crusted, ulcerated or atrophied. There is not usually any bleeding. Where the skin is involved, there may be radiating rhagades (linear fissures) from the corner of the mouth. Infrequently, the dermatitis (which may resemble eczema) can extend from the corner of the mouth to the skin of the cheek or chin. If "Staphylococcus aureus" is involved, the lesion may show golden yellow crusts. In chronic angular cheilitis, there may be suppuration (pus formation), exfoliation (scaling) and formation of granulation tissue.

Sometimes contributing factors can be readily seen, such as loss of lower face height from poorly made or worn dentures, which results in mandibular overclosure ("collapse of jaws"). If there is a nutritional deficiency underlying the condition, various other signs and symptoms such as glossitis (swollen tongue) may be present. In people with angular cheilitis who wear dentures, often there may be erythematous mucosa underneath the denture (normally the upper denture), an appearance consistent with denture-related stomatitis. Typically the lesions give symptoms of soreness, pain, pruritus (itching) or burning or a raw feeling.

Orofacial granulomatosis is enlargement of lips due to the formation of non-caseating granulomatous inflammation, which obstruct lymphatic drainage of the orofacial soft tissues, causing lymphedema. Essentially, granulomatous cheilitis refers to the lip swelling that accompanies this condition. "Median cheilitis" may be seen, which is fissuring in the midline of the lips due to the enlargement of the lips. Angular cheilitis may also be associated with orofacial granulomatosis.

A related condition is Melkersson–Rosenthal syndrome, a triad of facial palsy, chronic lip edema, and fissured tongue. "Miescher’s cheilitis", and "granulomatous macrocheilitis", are synonyms of granulomatous cheilitis.

Angular cheilitis (AC), is inflammation of one or both corners of the mouth. Often the corners are red with skin breakdown and crusting. It can also be itchy or painful. The condition can last for days to years. Angular cheilitis is a type of cheilitis (inflammation of the lips).

Angular cheilitis can be caused by infection, irritation, or allergies. Infections include by fungi such as "Candida albicans" and bacteria such as "Staph. aureus". Irritants include poorly fitting dentures, licking the lips or drooling, mouth breathing resulting in a dry mouth, sun exposure, overclosure of the mouth, smoking, and minor trauma. Allergies may include to substances like toothpaste, makeup, and food. Often a number of factors are involved. Other factors may include poor nutrition or poor immune function. Diagnosis may be helped by testing for infections and patch testing for allergies.

Treatment for angular cheilitis is typically based on the underlying causes along with the use of a barrier cream. Frequently an antifungal and antibacterial cream is also tried. Angular cheilitis is a fairly common problem, with estimates that it affects 0.7% of the population. It occurs most often in the 30s to 60s, although is also relatively common in children. In the developing world, iron and vitamin deficiencies are a common cause.

Chapped lips (also cheilitis simplex or common cheilitis) are characterized by cracking, fissuring, and peeling of the skin of the lips, and are one of the most common types of cheilitis. While both lips may be affected, the lower lip is the most common site. There may also be burning or the formation of large, painful cracks when the lips are stretched. If chronic, cheilitis simplex can progress to crusting and bleeding.

Lip licking, biting, or rubbing habits are frequently involved. Paradoxically, constant licking of the lips causes drying and irritation, and eventually the mucosa splits or cracks. The lips have a greater tendency to dry out in cold, dry weather. Digestive enzymes present in saliva may also irritate the lips, and the evaporation of the water in saliva saps moisture from them.

Some children have a habit of sucking and chewing on the lower lip, producing a combination of cheilitis and sharply demarcated perioral erythema (redness).

Treatment is usually successful with barrier lubricants, such as lip salve or Vaseline. Medical grade (USP) lanolin accelerates repair of the lips, and is used in some lip repair products.

Sometimes the term "cheilitis simplex" is used as a synonym for cheilitis generally, however, exfoliative cheilitis (discussed later) is also sometimes stated to be the equivalent of chapped lips.