The dynamic evaluation of tumor markers is a promising area of investigation which is expected to provide clinical information when serial samples are available from the same patient. This is feasible in the post-operatory evaluation, during the follow-up after the treatment for to the primary tumor and in the monitoring of the treatment for metastatic disease. Variations among serial samples may be assessed using both empirical and mathematical approaches. Empirical approaches rely on overcoming a given percentage usually chosen on the base of arbitrary decisions. Mathematical approaches include the actual half-life, the doubling time, a dose/time regression analysis and the calculation of the critical difference. The two former are currently used in clinical practice whereas the two latter are still matter of investigation. As concerns the assessment of the radicality of the surgery for the primary tumor, the serum markers are used in germ cell tumors and in prostate cancer. The half-life of the markers is the decision criteria used in germ cell cancers, while in prostate cancer PSA is expected to be undetectable more than 30 days after the radical prostatectomy. Tumor markers are currently used during the follow-up of several malignancies after the treatment for primary tumor. Although several samples are available, decision criteria are still based on positive/negative cut-off values in several instances. Promising dynamic approaches are under investigation and are expected to lead to earlier and probably more accurate information concerning the disease progression. A critical point still under debate is the actual impact of tumor markers on patients' survival in malignancies incurable when metastatic, such as colorectal cancer and breast cancer. This matter urgently demands perspective clinical studies. Finally, the dynamic use of tumor markers is now commonly applied in the monitoring of the therapy for metastatic malignancies. In this clinical setting mathematical criteria are used for ovarian and and germ cell tumors with promising results. Nevertheless, the use of empirical criteria, namely the percentage of variation between two consecutive samples, is successfully used for the monitoring of the therapy of metastatic breast cancer. In conclusion, when several samples are available from an individual patient they may be evaluated according to dynamic criteria instead of referring to a conventional positive/negative cut-off point. Although mathematical decision criteria are expected to provide more reliable data, empirical approaches are used as well and provide useful information in decision making.