Recent advances in the research of Nm23/NDP kinase, a tumor metastasis suppressor, are summarized in relation to tumor metastasis. The tumor metastatic potential of cancer cells is associated with decreased levels of Nm23/NDP kinase in animal model systems and human cases including breast cancer. One of the two isoforms seems to be responsible for metastatic processes. Whether this altered expression occurs at the transcriptional level or as a result of allelic deletion of the gene remains to be determined. The possible mechanism by which the Nm23/NDP kinase level regulates metastatic potential of cancer cells are discussed in view of its biochemical properties. The potential importance of the interaction between Nm23/NDP kinase and signal transducing systems is postulated in determining the metastatic potential of cancer cells.