Physicians are often faced with the dilemma of how to incorporate tumor markers into routine clinical decision-making. Tumor markers may influence clinical decisions at various stages of cancer therapy: screening, diagnosis, prognosis, detection of early relapse, and monitoring of therapy. We use the examples of beta-human chorionic gonadotrophin (beta-HCG) and alpha-fetoprotein (AFP) as markers for germ cell tumors (GCT), and prostate-specific antigen (PSA) as a marker for prostate cancer, to illustrate their use and limitations for these purposes. We then focus on monitoring and choice of treatment by presenting three vignettes; these highlight the potential benefits and problems associated with the use of tumor markers for monitoring and detection of early relapse in asymptomatic patients.