Embryologically, thymic epithelium is derived from both the ectoderm and the endoderm of the 3rd & 4th branchial cleft and pharyngeal pouches. Thymoma is a tumor arising from epithelium of thymus gland. Majority of thymoma looks histological benign. Usually it follows indolent course in patient surviving for years. 34% of thymomas invades their own capsules, extending to surrounding structures. Less than 10% behave like invasive epithelial malignancies. Incidence 0.15 cases per 100,000 accounting 0.2 to 1.5% of all malignancies Thymoma constitute about 50% of anterior mediastenal neoplasm in adult, Lymphoma -25% Peak age of incidence – 4th to 6th decades of life. Children very rarely affected.