Cancer is a leading cause of morbidity and death in solid organ transplant recipients. The risk of malignancy is increased over two-fold compared to the general population. This increased risk is attributed to multiple factors in this patient population, including underlying medical conditions, oncogenic viruses, and the use of immunosuppressive medications to prevent organ rejection. When cancer does occur in solid organ transplant recipients, it is often more aggressive, more difficult to treat, and has a poorer overall prognosis. This chapter reviews the risk of malignancy in solid organ transplant recipients.