Of the growing group of cancer biomarkers, circulating humoral tumour markers (TM) are the longest-known subgroup of substances with protein, lipid, glycolipid or carbohydrate structure, that can be used as probes for identifying the origin, growth and therapy-related debulking of a tumor. Relevant TM are well-defined and easy to determine, and disturbing factors and statistical characteristics well-known. They are most commonly used not so much for early detection, localization and diagnosis, but for treatment monitoring, detection of recurrence and, latterly, also for outcome prediction. Their particular value lies in their supporting role in invasive imaging or endoscopic procedures. In some cases, they may also prolong the intervals between for such procedures.