A number of biological markers have been recently introduced in clinical use as an aid for diagnosis and monitoring of malignant tumors. Some of them are relatively tumor-specific, i.e. CA 125 for non-mucinous ovarian cancer, CA 15.3 for breast cancer, PSA for prostate cancer, CA 19.9 for colo-rectal and pancreatic cancers. The clinical usefulness, the sensitivity and specificity of these tumor markers and of a few others (CA 50, SCC and TPA) are commented in this review.