Dental caries is a disease that damages tooth structures, resulting in what is commonly called tooth decay or cavities, which are holes in the teeth. This damage first affects the hard tissues of the teeth (enamel, dentin and cementum). As the destruction progresses, these tissues begin to break down, which can eventually lead to holes in the teeth. If left untreated, the disease can lead to pain, tooth loss, infection, and, in severe cases, death.There is a long history of dental caries: over a million years ago, hominids such as Australopithecus suffered from cavities. However, the incidence of cavities was very low well into the Paleolithic and Mesolithic periods. The largest increases in the prevalence of caries have been associated with dietary changes. Today, caries remains one of the most common diseases throughout the world.

There are numerous ways to classify dental caries. Although the presentation may differ, the risk factors and development among distinct types of caries remain largely similar. Initially, it may appear as a small chalky area but eventually develop into a large, brown cavitation. Though sometimes caries may be seen directly, radiographs are frequently needed to inspect less visible areas of teeth and to judge the extent of destruction.

Tooth decay is caused by certain types of acid-producing bacteria (specifically Lactobacillus species, Streptococcus mutans, and Actinomyces species) which cause damage in the presence of fermentable carbohydrates such as sucrose, fructose, and glucose. The resulting high levels of acidity from lactic acid in the mouth affect teeth because a tooth's special mineral content causes it to be sensitive to low pH. Specifically, a tooth (which is primarily mineral in content) is in a constant state of back-and-forth demineralization and remineralization between the tooth and surrounding saliva. When the pH at the surface of the tooth drops below 5.5, demineralization proceeds faster than remineralization (i.e. there is a net loss of mineral structure on the tooth's surface). This results in the ensuing decay. Depending on the extent of tooth destruction, various treatments can be used to restore teeth to proper form, function, and aesthetics, but there is no known method to regenerate large amounts of tooth structure. Instead, dental health organizations advocate preventive and prophylactic measures, such as regular oral hygiene and dietary modifications, to avoid dental caries.