Ian Tannock, MD, PhD, DSc, from the University of Toronto, Toronto, Canada, discusses the problems limiting the use of personalized medicine at the European Cancer Congress of the European Cancer Organisation (ECCO) 2017 in Amsterdam, Netherlands. Personalized medicine is currently typically defined as genetic analysis of a patient’s tumor, in order to match treatments to the mutations found. Prof. Tannock points out that for some mutations, such as HER2 in breast cancer, matching of treatments to mutations is very successful. However, for many mutations, no effective targeted agents are available. Furthermore, the targeted agents that can be used show similar toxicity levels as chemotherapy, often precluding the use of these treatments together. An additional problem is tumor heterogeneity, meaning that tens of different mutations can be found within the same patient in different parts of the primary tumor or in metastases. Prof. Tannock describes how because of this heterogeneity, targeted treatments may only work for a while before resistance is developed.