Toothlessness or edentulism is the condition of having no teeth. In organisms that naturally have teeth, it is the result of tooth loss. Organisms that never possessed teeth can also be described as edentulous. Examples are the members of the former zoological classification order of Edentata, which included anteaters and sloths, as they possess no anterior teeth and no or poorly developed posterior teeth.

In naturally dentate species, edentulism is more than just the simple presence or absence of teeth. It is biochemically complex because the teeth, jaws, and oral mucosa are dynamic. Processes such as bone remodeling (loss and gain of bone tissue) in the jaws and inflammation of soft tissue in response to the oral microbiota are clinically important for edentulous people. For example, bone resorption in the jaw is frequently how the teeth were able to detach in the first place. The jaw in an edentulous area undergoes further resorption even after the teeth are gone; and the insertion of dental implants can elicit new bone formation, leading to osseointegration. Meanwhile, bacteria and yeasts of the oral cavity and the immune system of their host create an immensely complicated and constantly changing interplay that presents clinically as gingivitis, caries, stomatitis, and other periodontal pathology.

Tooth regeneration is an ongoing stem cell–based field of study that aims to find methods to reverse the effects of decay; current methods are based on easing symptoms.

In humans, teeth serve to:

Beyond impairing physical function, tooth loss also has a psychological impact: it has been shown to generally lower a patient's quality of life, with this compromised oral function leading to decreased self-esteem and a decline in psychological well-being. Patients may be embarrassed to smile, eat and talk.

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 intercuspidation. 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.

As stated, the position of maximal closure in the presence of teeth is referred to as maximum intercuspidation, and the vertical jaw relationship in this position is referred to as the vertical dimension of occlusion. With the loss of teeth, there is a decrease in this vertical dimension, as the mouth is allowed to overclose when there are no teeth present to block the further upward movement of the mandible towards the maxilla. This may contribute, as explained above, to a sunken-in appearance of the cheeks, because there is now "too much" cheek than is needed to extend from the maxilla to the mandible when in an over closed position. If this situation is left untreated for many years, the muscles and tendons of the mandible and the TMJ may manifest with altered tone and elasticity.