Metastasis, the spread of neoplastic cells from a primary site to distant organs, is responsible for the majority of cancer deaths. Dissemination of malignant cells throughout the body and their survival to form secondary growths is a complicated process dependent on both host and tumor properties. Although a primary tumor may release many cells, only a few survive the interactions with host defense mechanisms to yield distant cancer growths. In recent years a considerable body of evidence has accumulated which suggests that the survival of these few cells is not a random event. Rather, it represents the selection of a pre-existent metastatic subpopulation of tumor cells within the parental population. Such a process has profound implications in determining approaches to therapy and to the elucidation of those tumor-cell properties which are responsible for successful metastatic spread. These implications are discussed within the framework of what is at present known about the pathogenesis of cancer metastasis.