This article addresses the structural and quantitative aspects of human tooth innervation and briefly considers the functions and clinical relevance of tooth axons. The classification of peripheral axons, the pulpal and dentinal innervation, and the theories of dentin sensitivity are discussed. Quantitative studies on tooth innervation are also reviewed. Human premolars receive about 2300 axons at the root-apex of which about 13% are myelinated and 87% are nonmyelinated fibers. Most apical myelinated axons are fast-conducting A delta-fibers with their receptive fields located at the pulpal periphery and inner dentin. These fibers are probably activated by a hydrodynamic mechanism and conduct impulses that are perceived as a short well-localized sharp pain. Most C-fibers are slow-conducting fine sensory afferents with their receptive fields located in the pulp and transmit impulses that are experienced as dull poorly localized and lingering pain. In addition to the nociceptive alarm signaling, the intradental sensory axons may play a regulatory role in the maintenance and repair of the pulpodentinal complex.