Circulating tumour cells (CTCs), as 'liquid biopsy”, has a major benefit over traditional tissue biopsy and has the potential to become a less invasive and more cost effective cancer biomarker. The presence of CTCs in the circulation indicates the presence of a tumour and the possibility of metastatic spread. Hence, the characterisation of CTCs is expected to provide crucial insights into the mechanisms of metastasis. It can also provide useful information about the future use of CTCs as a surrogate endpoint biomarker in diagnosis, prognosis, and treatment response prediction by minimizing the limitations of tissue biopsies. Also, it provides a new horizon for the development of novel targeted therapies. However, the lack of specific and effective methods is the key limitation in CTC detection and isolation in patients with cancer. Therefore, more responsive methods and approaches may be needed to improve the accuracy of CTC measurements. Herein, this book chapter will provide a current picture of CTCs as surrogate biomarkers for disease diagnosis, prognosis and predicting therapy response, along with the risk of relapse in cancers.