Diagnosis is usually made on the clinical appearance without the need for a tissue biopsy. However, when biopsies have been taken, the histologic appearance is one of marked elongation and hyperparakeratosis of the filiform papillae and numerous bacteria growing on the epithelial surface.

Hairy tongue may be confused with hairy leukoplakia, however the latter usually occurs on the sides of the tongue and is associated with an opportunistic infection with Epstein–Barr virus on a background immunocompromise (almost always human immunodeficiency virus infection but rarely other conditions which suppress the immune system).

The differential diagnosis includes oral lichen planus, erythematous candidiasis, leukoplakia, lupus erythematosus, glossitis, and chemical burns. Atrophic glossitis is usually distinguished from benign migratory glossitis on the basis of the migrating pattern of the lesions and the presence of a whitish border, features which are not present in atrophic glossitis, which instead shows lesions which enlarge rather than migrate. Rarely, blood tests may be required to distinguish from glossitis associated with anemia or other nutritional deficiencies. Since the appearance and the history of the condition (i.e. migrating areas of depapillation) are so striking, there is rarely any need for biopsy. When biopsy is taken, the histopathologic appearance is quite similar to psoriasis:

- Hyperparakeratosis.

- Acanthosis.

- Subepithelial T lymphocyte inflammatory infiltrate.

- Migration of neutrophilic granulocytes into the epithelial layer, which may create superficial microabscesses, similar to the Munro's microabscesses described in pustular psoriasis.

The diagnosis is usually made on the clinical appearance alone, and biopsy is not usually indicated. The histologic appearance is one of marked hyperparakeratosis producing a ragged surface with many projections of keratin. Typically there is superficial colonization by bacteria. There may be vacuolated cells in the upper portion of the prickle cell layer. There is a similarity between this appearance and that of hairy leukoplakia, linea alba and leukoedema. In people with human immunodeficiency virus, who are at higher risk of oral hairy leukoplakia, a tissue biopsy may be required to differentiate between this and frictional keratosis from cheek and tongue chewing.

Hairy tongue (lingua villosa) refers to a marked accumulation of keratin on the filiform papillae on the dorsal surface of the tongue, giving a hair-like appearance. Black tongue (lengua negra) refers to a black discoloration of the tongue, which may or may not be associated with hairy tongue. However, the elongated papillae of hairy tongue usually develop discoloration due to growth of pigment producing bacteria and staining from food. Hence the term black hairy tongue, although hairy tongue may also be discolored yellow or brown. Transient, surface discoloration that is not associated with hairy tongue can be brushed off. Drug-induced black hairy tongue specifically refers to BHT that develops because of medication.

Macroglossia is usually diagnosed clinically. Sleep endoscopy and imaging may be used for assessment of obstructive sleep apnea. The initial evaluation of all patients with macroglossia may involve abdominal ultrasound and molecular studies for Beckwith–Wiedemann syndrome.

Tongue lesions are very common. For example, in the United States one estimated point prevalence was 15.5% in adults. Tongue lesions are more common in persons who wear dentures and tobacco users. The most common tongue conditions are geographic tongue, followed by fissured tongue and hairy tongue.

The Kennedy classification quantifies partial edentulism. An outline is covered at the removable partial denture article.

Good oral hygiene (thorough tooth brushing and flossing and regular professional cleaning and examination) may be helpful to prevent these disorders. Drinking plenty of water and the production of enough saliva, aid in the reduction of bacterial growth. Minimizing irritants or injury in the mouth when possible can aid in the prevention of glossitis. Avoiding excessive use of any food or substance that irritates the mouth or tongue may also help.

The condition may disappear over time, but it is impossible to predict if or when this may happen.

The lesions are harmless, and no treatment is indicated beyond reassurance, unless the person requests it. The most common and simple treatment is construction of a specially made acrylic prosthesis that covers the biting surfaces of the teeth and protects the cheek, tongue and labial mucosa (an occlusal splint). This is either employed in the short term as a habit breaking intention, or more permanently (e.g. wearing the prosthesis each night during sleep). Psychological intervention is also reported, but does not appear to be beneficial.

The diagnosis is usually made on the clinical appearance, and tissue biopsy is not usually needed. The histologic picture is one of superficial candidal hyphal infiltration and a polymorphonuclear leukocytic inflammatory infiltrate present in the epithelium. The rete ridges are elongated and hyperplastic (pseudoepitheliomatous hyperplasia, which may be mistaken for carcinoma).

Oral and maxillofacial pathology, previously termed oral pathology, is a speciality involved with the diagnosis and study of the causes and effects of diseases affecting the oral and maxillofacial regions (i.e. the mouth, the jaws and the face). It can be considered a speciality of dentistry and pathology. Oral pathology is a closely allied speciality with oral and maxillofacial surgery and oral medicine.

The clinical evaluation and diagnosis of oral mucosal diseases are in the scope of oral & maxillofacial pathology specialists and oral medicine practitioners, both disciplines of dentistry.

When a microscopic evaluation is needed, a biopsy is taken, and microscopically observed by a pathologist. The American Dental Association uses the term oral and maxillofacial pathology, and describes it as "the specialty of dentistry and pathology which deals with the nature, identification, and management of diseases affecting the oral and maxillofacial regions. It is a science that investigates the causes, processes and effects of these diseases."

In some parts of the world, oral and maxillofacial pathologists take on responsibilities in forensic odontology.

A diagnosis of hyposalivation is based predominantly on the clinical signs and symptoms. There is little correlation between symptoms and objective tests of salivary flow, such as sialometry. This test is simple and noninvasive, and involves measurement of all the saliva a patient can produce during a certain time, achieved by dribbling into a container. Sialometery can yield measures of stimulated salivary flow or unstimulated salivary flow. Stimulated salivary flow rate is calculated using a stimulant such as 10% citric acid dropped onto the tongue, and collection of all the saliva that flows from one of the parotid papillae over five or ten minutes. Unstimulated whole saliva flow rate more closely correlates with symptoms of xerostomia than stimulated salivary flow rate. Sialography involves introduction of radio-opaque dye such as iodine into the duct of a salivary gland. It may show blockage of a duct due to a calculus. Salivary scintiscanning using technetium is rarely used. Other medical imaging that may be involved in the investigation include chest x-ray (to exclude sarcoidosis), ultrasonography and magnetic resonance imaging (to exclude Sjögren's syndrome or neoplasia). A minor salivary gland biopsy, usually taken from the lip, may be carried out if there is a suspicion of organic disease of the salivary glands. Blood tests and urinalysis may be involved to exclude a number of possible causes. To investigate xerophthalmia, the Schirmer test of lacrimal flow may be indicated. Slit-lamp examination may also be carried out.

There are no specific treatments for this problem, other than using ice or numbing medicines to ease the pain.

In traditional Chinese medicine, scalloping of the tongue is said to indicate qi vacuity. In some homeopathic sources, scalloping of the tongue is said to be indicative of high blood pressure.

BMS is a diagnosis of exclusion, i.e. all other explanations for the symptoms are ruled out before the diagnosis is made. There are no clinically useful investigations that would help to support a diagnosis of BMS (by definition all tests would have normal results), but blood tests and / or urinalysis may be useful to rule out anemia, deficiency states, hypothyroidism and diabetes. Investigation of a dry mouth symptom may involve sialometry, which objectively determines if there is any reduction of the salivary flow rate (hyposalivation). Oral candidiasis can be tested for with use of a swabs, smears, an oral rinse or saliva samples. It has been suggested that allergy testing ("e.g.", patch test) is inappropriate in the absence of a clear history and clinical signs in people with a burning sensation in the mouth. The diagnosis of a people with a burning symptom may also involve psychologic screening e.g. depression questionnaires.

The second edition of the International Classification of Headache Disorders lists diagnostic criteria for "Glossodynia and Sore Mouth":

It is a relatively common condition, with an estimated prevalence of 6.8%–11% amongst children. Males are more commonly affected. The condition may be seen at any age, but generally affects older people more frequently. The condition also generally becomes more accentuated with age. The prevalence of the condition increases significantly with age, occurring in 40% of the population after the age of 40.

Tongue diseases can be congenital or acquired, and are multiple in number. Considered according to a surgical sieve, some example conditions which can involve the tongue are discussed below. Glossitis is a general term for tongue inflammation, which can have various etiologies, e.g. infection.

Treatment and prognosis of macroglossia depends upon its cause, and also upon the severity of the enlargement and symptoms it is causing. No treatment may be required for mild cases or cases with minimal symptoms. Speech therapy may be beneficial, or surgery to reduce the size of the tongue (reduction glossectomy). Treatment may also involve correction of orthodontic abnormalities that may have been caused by the enlarged tongue. Treatment of any underlying systemic disease may be required, e.g. radiotherapy.

Anti-tumour necrosis factor α antagonists (e.g. infliximab)

Dietary restriction of a particular suspected or proven antigen may be involved in the management of OFG, such as cinnamon or benzoate-free diets.

The diagnosis is usually made by tissue biopsy, however this cannot reliably distinguish between the granulomas of OFG and those of Crohn's disease or sarcoidosis. Other causes of granulomatous inflammation are ruled out, such as sarcoidosis,

Crohn's disease, allergic or foreign body reactions and mycobacterial infections.

Crenated tongue (also called scalloped tongue, pie crust tongue, lingua indentata, or crenulated tongue) is a descriptive term for the appearance of the tongue when there are indentations along the lateral borders (the sides), as the result of compression of the tongue against the adjacent teeth.

The oral mucosa in the area of crenation is usually of normal color, but there may be erythema (redness) if exposed to a high degree of friction or pressure. Crenated tongue is usually asymptomatic and harmless.

It is not a disease as such, but usually results from habits where the tongue is pressed against the lingual surfaces (the side facing the tongue) of the dental arches, or from any cause of macroglossia (enlarged tongue), which in itself has many causes such as Down syndrome.

Where crenation is caused by parafunctional habits, there may also be associated bruxism, linea alba, or morsicatio buccarum.

Strawberry tongue (also called raspberry tongue), refers to glossitis which manifests with hyperplastic (enlarged) fungiform papillae, giving the appearance of a strawberry. White strawberry tongue is where there is a white coating on the tongue through which the hyperplastic fungiform papillae protrude. Red strawberry tongue is where the white coating is lost and a dark red, erythematous surface is revealed, interspaced with the hyperplastic fungiform papillae. White strawberry tongue is seen in early scarlet fever (a systemic infection of group A β- hemolytic streptococci), and red strawberry tongue occurs later, after 4–5 days. Strawberry tongue is also seen in Kawasaki disease (a vasculitic disorder primarily occurring in children under 5), and toxic shock syndrome. It may mimic other types of glossitis or Vitamin B12 deficiency.

BMS is benign (importantly, it is not a symptom of oral cancer), but as a cause of chronic pain which is poorly controlled, it can detriment quality of life, and may become a fixation which cannot be ignored, thus interfering with work and other daily activities. Two thirds of people with BMS have a spontaneous partial recovery six to seven years after the initial onset, but in others the condition is permanent. Recovery is often preceded by a change in the character of the symptom from constant to intermittent. No clinical factors predicting recovery have been noted.

If there is an identifiable cause for the burning sensation (i.e. primary BMS), then psychologic dysfunctions such as anxiety and depression often disappear if the symptom is successfully treated.

Chronic ulcerative stomatitis is a recently discovered condition with specific immunopathologic features. It is characterized by erosions and ulcerations which relapse and remit. Lesions are located on the buccal mucosa (inside of the cheeks) or on the gingiva (gums). The condition resembles Oral lichen planus when biopsied.

The diagnosis is made with Immunofluorescence techniques, which shows circulating and tissue-bound autoantibodies (particulate stratified squamous-epithelium-specific antinuclear antibody) to DeltaNp63alpha protein, a normal component of the epithelium. Treatment is with hydroxychloroquine.