Analysis of circulating tumour DNA (ctDNA) in liquid biopsies is an invaluable new tool for the management of cancer. However, ctDNA is often present as a small fraction of the circulating free DNA. As a consequence, most technologies used to analyse tissue biopsies are unsuitable because of their limited sensitivity. Droplet digital PCR (ddPCR) has proven capable of meeting many of the challenges in assessing ctDNA showing both high sensitivity and specificity. A particular advantage of ddPCR is a rapid turnaround time with urgent results being available in less than 6 hours after receipt of a blood sample. A second advantage is absolute quantification of templates which facilitates monitoring of response to therapy and minimal residual disease. However, the identification of a truncal tumour specific change is a necessity. Driver mutations are ideal but chromosome rearrangements and DNA methylation changes are potential alternatives.