Most cancer deaths are due to metastasis – the spread of cancer from its site of origin to distant, vital organs, and the physiological damage caused by tumor growth in those organs. While the broad outlines of the process of metastatic spread are known, much of the details of the process remain poorly understood. To continue to improve cancer survival rates, we must face and tackle the problems inherent to metastatic disease. Cancers that are detected early, before they are believed to have spread to other organs, are generally treated with more success than cancers that are metastatic at diagnosis. However, even cancers that are detected early will recur in some patients, but our ability to predict which individuals will have recurrences is limited. Thus, adjuvant therapy is often given to patients with early stage disease who are believed as a group to be risk for recurrence, leading to over-treatment of some patients to benefit a subset of them. Some recurrences can occur years or even decades after apparently successful primary treatment, and research on tumor dormancy is providing insights into these delayed recurrences.