Clinical cancer dormancy is evident from the detection of circulating tumor cells in the blood and tissue-residing disseminated tumor cells in the bone marrow of cancer survivors who have been clinically disease free. Emerging evidence from clinical and preclinical studies suggests that tumor dormancy is a critical step in the development of both primary cancer and advanced-stage disease. In this review, it is shown that (i) naturally occurring tumor dormancy precedes occurrence of primary cancer, and (ii) conventional cancer therapies result in treatment-induced tumor dormancy, which in turn could lead to distant recurrence of cancer or permanent tumor dormancy, depending on immunogenic status of dormancy. Given that cellular dormancy is an evolutionary conserved survival mechanism in biologic systems, any stress or cytotoxic therapy could trigger cellular dormancy. Therefore, a successful cancer therapy is likely to be achieved by establishing permanent tumor dormancy and preventing distant recurrence of cancer or by eliminating dormant tumor cells. This could be accomplished by cancer immunotherapy because of the establishment of long-term memory responses. Cancer Res; 74(23); 6745–9. ©2014 AACR.