Cancer Markers are substances found in the blood, urine, or body tissues that can be elevated in cancer, among other tissue types. There are many different cancer markers, each indicative of a particular disease process, and they are used in oncology to help detect the presence of cancer. Tumor markers are molecules occurring in blood or tissue that are associated with cancer and whose measurement or identification is useful in patient diagnosis or clinical management. The ideal marker would be a "blood test" for cancer in wich a positive result would occur only in patients with malignancy, one that would correlate with stage and response to treatment and that was easily and reproducibly measured. No tumor marker now available has met this ideal. Tumor markers can be used for one of four purposes: (1) screening a healthy population or a high risk population for the presence of cancer; (2) making a diagnosis of cancer or of a specific type of cancer; (3) determining the prognosis in a patient; (4) monitoring the course in a patient in remission or while receiving surgery, radiation, or chemotherapy. No test meets all of those requirements. Specifically, no marker has been established as a pratical cancer screening tool either in a general healthy population or in most high risk poulations. The reason for this is the relative lack of sensitivity and specificity of the available tests, given the low prevalence of cancers in most population groups. Given the low prevalence of cancer in general, even tests that are highly sensitive and specific may have low predictive values. Tumor markers include many substances that are not readily systematically organized.Those discussed here are generally products or the cancer cell, although none is unique to cancer cells; they represent aberrant tumor production of a normal element. Some markers are produced by the organism in response to the cancer''s presence.