Secondary Neoplasia (SN) after HCT belong to the most feared long-term complications. They include any malignant disorder occurring after HCT. There are three types of SN: therapy-related myeloid neoplasms, occurring mainly after auto-HCT; donor-derived malignancies after allo-HCT; and second solid neoplasms after either auto- or allo-HCT. Many of these SN have a higher incidence compared to the general population. In this chapter, pathophysiology issues, risk factors, screening and management recommendations are discussed. Since SN can occur even decades after HCT, life-long surveillance is needed.