Thymoma is the most common primary neoplasm of the tymus. The majority of thymomas are encapsulated masses and exhibit benign behavior. Less frequently they may be invasive, or rarely they may metastasize to distant sites. The usual clinical presentation is that of an anterior mediastinal mass found accidentally in an asymptomatic patient. Computed tomography and magnetic resonance may be helpful in the evaluation of adjacent structures. The histologic classification proposed by Muller-Hermelink is useful in predicting and defining the risk of invasiveness of thymomas showing a significant correlation between tumor cell type and stage. The treatment of choice is complete surgical excision; radiation therapy may be used adjunctively to surgery in the treatment of invasive tumors. The choice of surgical approach is conditioned by site and grading of thymoma. The prognosis of encapsulated thymoma is generally favorable; invasive tumors are associated with a worse prognosis but may respond to radical resection.