As survival after a diagnosis of cancer improves, characterization of the late sequelae of treatment becomes critical. The development of second malignant neoplasms represents one of the most serious side effects of treatment with radiation and chemotherapy. Although secondary leukemia was the first reported carcinogenic effect resulting from cancer treatment, solid tumors now comprise the largest second tumor burden in some populations of survivors. It should be recognized, however, that solid cancers do not necessarily represent an adverse effect of therapy, but may also reflect the operation of shared etiologic factors, host determinants, gene-environment interactions, and other influences. Quantification of second cancer risk is important in terms of patient management, enabling clinicians to make informed decisions with regard to optimal treatment of the initial cancer, balancing efficacy against acute and chronic sequelae. This article focuses on selected highlights and recent developments in treatment-associated solid malignancies, with emphasis on radiotherapy and chemotherapy in adults, and summarizes areas for future research. Although cancer therapy represents a double-edged sword, it should always be recognized that it is advances in treatment that are largely responsible for the tremendous improvement in patient survival. Thus, the benefit derived from many cancer therapies far outweighs any risk of developing a second cancer.