Neuroendocrine tumors (NETs) of the mediastinum and thymus are rare and are usually discovered incidentally. Rosai and Higa were the first to acknowledge the existence of carcinoid tumors in the thymus and to separate them from more common tumors arising in this location, such as thymoma. The source of the primary NETs of the thymus has been proposed that they are derived from Kulchitsky cells. In this chapter we discussed the World Health Organization (WHO) classification of the NETs. We also spotlighted on some chromosomal imbalances that have been found by investigators in the most recent molecular biology literatures of the thymic NETs, as gain of chromosome (Xp, 7p, 7q, 11q, 12q, 20q, and 19p) and loss (6q, 6p, 4q, 3p, 10q, 11q, and 13q). Losses of heterozygosity (LOH) at chromosome 1p and loss of chromosomes 3, 9p21, and Y have been also reported. At the end of this chapter, we discussed the up-to-date prognosis, management, response evaluation, and follow-up of the patient.