Personalising cancer medicine depends upon the implementation of personalised diagnostics and therapeutics. Detailed genomic screening is likely to play a central role in this. As the range of drugs and other therapies for cancer continues to increase, there is an increasingly urgent need for sensitive and specific measures of disease burden to guide treatment regimens. The ability to quantify disease burden with high accuracy and sensitivity in patients with cancer would open many potential routes to personalising therapeutic choices. For example, the intensity of therapy could be guided by the amount of disease at diagnosis; monitoring the response of patients to drugs could allow extension of the period of treatment in responders or early changeover of therapy in nonresponders; and early prediction of recurrence could allow salvage therapy to be instituted before complications of relapse develop. The detection of tumour-specific rearrangements in DNA free in the serum or plasma may provide a substantial advance in the accuracy of monitoring disease burden in patients with solid tumours.