Knowledge of caries bacteria and the inflammatory responses they elicit in the dental pulp is prerequisite to our understanding of the pathogenesis of pulpitis. Recent advances in immunology and neurophysiology can now explain some of the clinical manifestations of pulpitis. The purpose of this review is twofold. The first purpose is to review the literature of the caries microflora, the host immune responses they elicit, and how they do so. The relationship between both proinflammatory and anti-inflammatory cytokines and pulpitis is discussed. The proinflammatory properties of lipoteichoic acid, which is a common virulence factor among Gram-positive bacteria such as those found among the caries bacteria, are reviewed. The second purpose is to review how bacteria and their metabolites, as well as pulpal immune and inflammatory reactions to them, modify the pain sensation in pulpitis.