The mechanisms by which pain develops in carious teeth are poorly understood. The variability of dentin and pulp responses to carious attack is probably the cause for the variations in the pain symptoms presented by patients. This article is a review of the available data regarding the changes that dentin and pulp undergo in carious teeth. Such changes are discussed in relation to their possible influence on intradental sensory nerve excitability. Special emphasis is given to the various bacterial metabolites produced and accumulated in carious dentin and their ability to modulate pain symptoms from carious teeth.