Periodontitis is caused by extension of inflammation from the gingiva into deeper periodontal tissues. However, periodontitis can also be caused by pulpal infections that have entered the periodontal ligament either through the apical foramen or through the lateral canals. Such a periodontal lesion is termed as \"retrograde periodontitis\". Similarly, \"retrograde pulpitis\" can also occur as a result of periodontal disease and periodontal treatment. This chapter covers the pathways of communication between pulp and periodontium, effects of pulpal disease on the periodontium, effects of periodontitis on the dental pulp, and endoperio lesions.