The diagnosis and treatment of cancer presents a physical and mental burden to the patient, often involving diagnostic biopsies and surgeries or chemotherapeutic approaches with severe side-effects. Advances which enable early detection of cancer and close monitoring of the disease course without invasive procedures, and which can underpin a tailored approach to treatment, can therefore make a big difference to the quality of life of patients. Liquid biopsies can be used to access tumor cells and tumor DNA circulating in the blood. Monitoring these species can provide a minimally invasive and repeatable means to detect cancer, or gain information about its response to treatment.