Periapical lesions are among the most frequently diagnosed apical odontogenic pathologies in human teeth. The condition is generally described as apical periodontitis. Apical periodontitis is a sequel to endodontic infection and manifests itself as the host defense response to microbial challenge emanating from the root canal system to the periapical tissue. It is viewed as a dynamic encounter between microbial factors and host defenses at the interface between infected radicular pulp and periodontal ligament that results in local inflammation, resorption of hard tissues, destruction of other periapical tissues, and eventual formation of various histopathological categories of apical periodontitis, commonly referred to as periapical lesions. There are also factors located within the inflamed periapical tissue that can interfere with post-treatment healing of the lesion. The purpose of this article is to provide a comprehensive overview of the etiopathogenesis of apical periodontitis and causes of failed endodontic treatment. This study presents a histopathological analysis through optical microscopy of periapical lesions, commonly referred to as solid dental or periapical granuloma.