About 3% of all cancer patients suffer from cancer of unknown primary origin. These patients present with metastatic disease for which a primary site cannot be detected at the time of diagnosis. Sophisticated diagnostic techniques and operational procedures have failed to improve the diagnostic efficacy in this group of patients. Consequently, a limited diagnostic procedure with basic laboratory tests and imaging studies is sufficient for the diagnosis of this syndrome. The use of immunohistochemistry, as well as serum tumor markers of high specificity that may help to identify other tumors, is highly suggested. Although the prognosis for the majority of these patients still remains poor, several subsets of favorable outcome to treatment have been recognized. Nevertheless, promising in vitro data and new drugs on trials, paralleled with a better knowledge of the underlying pathogenetic molecular mechanisms, offer a more optimistic look to the future therapeutic management of these patients.