Radicular cyst is a pathologic cavity partially or completely lined by epithelium in an area of apical periodontitis that is the result of root canal infection of a tooth. The radicular cysts are presumably formed by inflammatory proliferation of epithelial cell rests of Malassez in the inflamed periapical tissues. Although several theories have been proposed as possible causes of cyst formation, the exact mechanism is still not clear. The prevalence of radicular cysts ranges from 15 to 20 % of all endodontic periapical osteolytic lesions. Histologically, radicular cysts can be categorized into pocket and true cyst. The radicular cyst appears as an osteolytic periapical radiolucent lesion around the apex of an endodontically involved tooth on conventional radiography and cone beam computed tomography. It is impossible to diagnose a radicular cyst clinically. The definitive diagnosis of a radicular cyst can only be made through histological examination of biopsy specimens. Therefore, all cyst-like periapical lesions associated with endodontically involved teeth should be initially treated with non-surgical root canal therapy. Because radicular cysts are caused by root canal infection, it is believed that pocket type of radicular cyst can be treated by conventional root canal therapy, however true type of radicular cysts may have to be managed surgically. Recurrence of radicular cysts is rare unless the root canal infection of a tooth persists because radicular cysts are not self-sustaining lesion such as developmental or neoplastic cysts. The prognosis of radicular cysts is favorable after endodontic treatment.