Tumour markers are substances developed in or induced by tumour cells and secreted into body fluids in which they can be quantified by non-invasive analyses. The malignant transformation of cells leads to increased concentrations of tumour markers and thus they can indicate malignant diseases. It appears, however, that other proliferative processes, i.e. inflammatory and benign transformations are also able to induce the rise of tumour marker levels. Due to their low sensitivity and specificity, tumour markers--except for PSA--are not useful in diagnosis and screening. Though disseminated malignant disorders are associated with high tumour marker levels, a correlation between their concentration and the tumour volume is not clearly approved. The use of tumour markers seems established for the follow-up after curative surgery and for the treatment and monitoring of palliative therapy.