This chapter addresses the etiological factors and the pathological processes involved in the development of pulpitis and apical periodontitis. Coronal sources of pulpal irritation such as caries, coronal cracks and fractures, attrition, abrasion erosion, or congenital defects in the crown lead to pulp inflammation. Both the pulp and the periapical tissues undergo a stage of inflammatory changes, before becoming physically infected by bacteria. The dental pulp is well equipped to detect invading pathogens and to mount an immune response to them. As with all other tissues in the body, the defense system of the pulp can be classified into the innate and the adaptive immune responses. The histopathological classification of apical periodontitis has historically been of great importance, as it was thought to influence the prognosis of non-surgical root canal treatment. Apical granulomas are the primary lesions involved in generating the apical immune response.