The thymus gland consists predominantly of epithelial cells and lymphocytes. When precursor leukocyte cells reach the thymus, they differentiate into T-lymphocytes. Thymoma is a neoplasm which originates within the epithelial cells of the thymus. This neoplasm constitutes 25% of all mediastinal tumors, therefore it is considered as the most common neoplasm of the anterior mediastinum. About one-third of patients with thymoma are asymptomatic. Another one-third present with local symptoms such as cough, chest pain, shortness of breath, hoarseness and trouble swallowing that indicate the spread of the tumor to the surrounding structures. The remaining one-third show systemic symptoms related to myasthenia gravis, such as muscle weakness. Thymoma can be classified into four stages that proper treatment can be done according to them. Stage-I represents the tumor within the thymus gland only. Stage-II occurs when the tumor spreads to the surrounding tissues. If the tumor invades the nearby organs, thymoma will be considered as stage-III thymoma. Finally, stage-IV describes the condition of deep spreading of tumor to other body parts. Males and females are equally affected. Mostly, thymoma occurs in the fourth and fifth decades of life. Radiography is the basic diagnostic tool for thymoma. Usually, surgical removal of the affected parts of the thymus gland (thymomectomy) is the best choice for treatment.