Hyperdontia is the condition of having supernumerary teeth, or teeth that appear in addition to the regular number of teeth (32 in the average adult). They can appear in any area of the dental arch and can affect any dental organ. The opposite of hyperdontia is hypodontia, where there is a congenital lack of teeth, which is a condition seen more commonly than hyperdontia. The scientific definition of hyperdontia is "any tooth or odontogenic structure that is formed from tooth germ in excess of usual number for any given region of the dental arch." The additional teeth, which may be few or many, can occur on any place in the dental arch. Their arrangement may be symmetrical or non-symmetrical.

The presence of a supernumerary tooth, particularly when seen in young children, is associated with a disturbance of the maxillary incisor region. This commonly results in the impaction of the incisors during the mixed dentition stage. The study debating this also considered many other factors such as: the patient's age, number, morphology, growth orientation and position of the supernumerary tooth. Alongside this issue, the presence of an extra tooth can impede the eruption of adjacent additional or normal teeth. Therefore, the presence of a supernumerary tooth when found must be approached with the appropriate treatment plan, incorporating the likelihood of incisal crowding. In some individuals, the additional teeth can erupt far from the dental arch, within the maxillary sinus. The extra teeth may also migrate to a different location after development. In some cases, supernumerary teeth can lead to the formation of cysts. Crowding is also frequently seen in people with hyperdontia.

There is evidence of hereditary factors along with some evidence of environmental factors leading to this condition. While a single excess tooth is relatively common, multiple hyperdontia is rare in people with no other associated diseases or syndromes. Many supernumerary teeth never erupt, but they may delay eruption of nearby teeth or cause other dental or orthodontic problems. Molar-type extra teeth are the most common type. Dental X-rays are often used to diagnose hyperdontia.

It is suggested that supernumerary teeth develop from a third tooth bud arising from the dental lamina near the regular tooth bud or possibly from splitting the regular tooth bud itself. Supernumerary teeth in deciduous (baby) teeth are less common than in permanent teeth.

Specific genes play a role in determining the number and pattern of teeth that develop in an individual, and mutations or variations in these genes can result in the formation of extra teeth. An individual can inherit a genetic predisposition to hyperdontia from its parents. If an individual with hyperdontia reproduces, their offspring have an increased likelihood of having the same condition, as their offspring have an increased likelihood of having the same condition because hyperdontia has been proposed as having an autosomal dominant mode of inheritance. This is the result of mutations that can be passed on to offspring through several different genes associated with tooth development. The MSX1 and MSX2 genes are among the most well-documented genetic factors associated with hyperdontia. Because these genes play a crucial role in tooth development and patterning, mutations in MSX1 and MSX2 can lead to the formation of extra teeth or changes in tooth number and shape. Additionally, mutations in the AXIN2 gene have been linked to hyperdontia and other anomalous dental traits. The AXIN2 gene is involved in regulating the Wnt signaling pathway, which plays a role in tooth development. Therefore, variations in this gene can disrupt tooth development and result in supernumerary teeth. PAX9 is another gene important to tooth development, and mutations in this gene have been associated with dental anomalies, including hyperdontia. PAX9 is involved in the formation of molar teeth, and disruptions in its function can lead to extra molars or changes in tooth morphology.

The formation of teeth begins during embryonic development and is a process that is highly regulated by various signaling pathways, which, if disrupted as a result of genetic mutations or environmental factors, can lead to developmental anomalies, including hyperdontia. Tooth development begins with the formation of tooth buds in the jaw. The dental lamina is a band of tissue in the developing oral cavity that gives rise to tooth buds. Hyperactivity of the dental lamina, as well as disruption of the differentiation and morphogenesis stages of tooth development, can lead to the formation of extra tooth buds, which can develop into supernumerary teeth. Furthermore, some developmental syndromes or medical conditions may also result in hyperdontia. For instance, cleidocranial dysplasia (CCD) is a genetic disorder that affects skeletal and dental development and is associated with the presence of supernumerary teeth. Trauma or injury to the oral cavity, particularly during tooth development, also has the potential to trigger the formation of supernumerary teeth because it disrupts normal tooth development. Occasionally, additional teeth may also arise from developmental anomalies like fusion or gemination. Fusion occurs when two tooth buds fuse together, creating a single, larger tooth. Gemination involves the incomplete division of a single tooth bud into two teeth. In some cases, these anomalies may take the form of the appearance of extra teeth.

Evolutionarily, hyperdontia can be seen as a result of genetic variation. Although extra teeth may be a maladaptive trait in modern humans due to potential dental issues and crowding, whether individuals with supernumerary teeth have a benefit or disadvantage is unknown. In the context of human evolutionary history, dental care was virtually nonexistent, meaning that there was no way to address dental issues such as tooth decay, infection, or loss of teeth. Therefore, individuals with supernumerary teeth may have been better equipped to cope with dental problems, for the loss of necessary teeth could be alleviated by having extra teeth present so that chewing and processing food remained possible. Furthermore, ancestral diets were often tougher and required more extensive chewing, meaning that being in the possession of extra teeth could help to facilitate the consumption of fibrous plant material and raw foods.

Hyperdontia is seen in a number of disorders, including Gardner's syndrome and cleidocranial dysostosis, where multiple supernumerary teeth develop.