Colorectal cancer is one of the most common cancers in the Western world and many patients develop distant metastases, most frequently in the liver. When possible, curative treatment of liver metastases is performed, often by a partial resection of the liver. However, for most patients with metastatic disease palliative systemic treatment is the only therapeutic option. This palliative systemic therapy prolongs the median overall survival of the patient population as a whole, but, in fact, only a subset of patients responds to this potentially toxic and expensive treatment.