An increase of targeted anticancer therapies has led to the beginning of a new era of cancer treatment, partly by replacing classical chemotherapies, partly supplementing these. Whereas for some substances only clinical experiences are relevant for treatment decisions, for some major cancer groups predictive markers are known that indicate probable tumor responses. To identify the latter, a need for companion diagnostics is given, often already existing as successful cooperation between pharmaceutical and diagnostic industries. This editorial focuses on the impact of companion diagnostic tests in personalized anticancer medicine, reporting recent advances in identifying and characterizing tumor subgroups responding to selected drugs. The most successful targeted therapies are directed against the EGFR/Her-2/neu receptors with regard to their downstream molecules in major cancer groups, including breast, gastric, lung and colorectal carcinomas. The development of biomarkers provides great opportunities to identify subpopulations with differential drug responses. On the one hand patients themselves are gaining major advantages of personalized and better tolerable cancer treatment, on the other hand, owing to very focused targeted therapies, these developments make possible cost-intensive targeted drug investigations and trials, especially in a situation of limited healthcare budgets.