This chapter discusses diseases of the periradicular tissues. Pulpal inflammation or infection can lead to symptomatic or asymptomatic apical periodontitis, acute alveolar abscess, phoenix abscess, or other conditions. Symptomatic apical periodontitis is a painful inflammation of the periradicular tissues caused by trauma, irritation or infection through the root canal. It may occur in vital or non-vital teeth and causes clinical symptoms like pain from biting or percussion. The chapter classifies periradicular diseases and discusses their causes and sequelae.