Pulpitis is inflammation of dental pulp tissue. The pulp contains the blood vessels, the nerves, and connective tissue inside a tooth and provides the tooth's blood and nutrients. Pulpitis is mainly caused by bacterial infection which itself is a secondary development of caries (tooth decay). It manifests itself in the form of a toothache.

Increased sensitivity to stimuli, specifically hot and cold, is a common symptom of pulpitis. A prolonged throbbing pain may be associated with the disease. However, pulpitis can also occur without any pain.

Reversible pulpitis is characterised by intermittent, brief discomfort initiated by a hot, cold or sweet stimulus. The pain evoked is of short duration and there is no lingering or spontaneous pain. The pain ceases within a short period after removal of the stimulus. With a reversible pulpitis, sleep is usually not affected and no analgesics are necessary.  Usually, no atypical change is evident on the radiograph. Pulp vitality tests are positive and it is possible to preserve a healthy vital pulp.

Irreversible pulpitis, in contrast, is characterised by a constant severe pain that arises without provocation. Characteristics may include sharp pain upon thermal stimulus, lingering thermal pain, spontaneity, and referred pain. Sometimes, the pain may be accentuated by postural changes such as lying down or bending over. If a stimulus is applied, the pain persists for minutes or hours after removal of the stimulus. These indicate that the vital inflamed pulp is not capable of healing and it is often indicated for the pulp to be removed as it is too damaged. Sleep may be disturbed and over-the-counter painkillers are often taken in an attempt to provide relief, but are usually ineffective.

Pulpitis may be caused by bacteria from dental caries that penetrate through the enamel and dentin to reach the pulp, or it may be mechanical, a result of trauma, such as physical damage to the tooth.

Inflammation is commonly associated with a bacterial infection but can also be due to other insults such as repetitive trauma or in rare cases periodontitis. The inflammation of dental pulp is mainly caused by an opportunistic infection of the pulp by a commensal oral microorganism. To reach the pulp, the most common route of the microorganism is through dental caries as well as from trauma, dentinal cracks and exposed dentin. Exposed dentin gives the microorganisms access to the pulp of the tooth through the dentinal tubules. In the case of penetrating decay, the pulp chamber is no longer sealed off from the environment of the oral cavity.

When the pulp becomes inflamed, pressure begins to build up in the pulp cavity, exerting pressure on the nerve of the tooth and the surrounding tissues. Pressure from inflammation can cause mild to extreme pain, depending upon the severity of the inflammation and the body's response. Unlike other parts of the body where pressure can dissipate through the surrounding soft tissues, the pulp cavity is very different. It is surrounded by dentin, a hard tissue that does not allow for pressure dissipation, so increased blood flow, a hallmark of inflammation, will cause pain.

The infection can also come from the apical foramen of the root. Cells in the dental pulp trigger an immune response from the invasion of foreign microorganisms. The inflammation of the pulp is a side effect of the immune response and causes pain.