Circulating tumour cells (CTCs) represent a unique and dynamic aspect of cancer progression, offering valuable insights into diagnosis, prognosis, disease monitoring, drug selection, and treatment response assessment. While conventional methods rely on physical markers such as size and density to isolate CTCs, the journey of CTC detection has evolved. Early techniques involving cell lysis and mRNA expression have evolved into approaches that leverage surface markers, enabling the capture of intact CTCs. For instance, EpCAM (epithelial cell adhesion molecule) is a well-established marker expressed on CTCs, widely adopted in the FDA-approved CellSearch® system to detect and capture CTCs. Notably, CTCs are closely linked with the metastatic spread of cancer. Studying the phenotypes of CTCs, such as clustering, epithelial-to-mesenchymal transition, and stemness, enhances the understanding of their behaviours. This chapter aims to provide a comprehensive knowledge of CTC, underscoring its potential as a non-invasive cancer biomarker. We address the challenges in isolating the entire CTC population, the limited sensitivity of current detection and isolation approaches, as well as the ongoing efforts to unlock their full clinical potentials. Additionally, the promising role of CTCs in advancing precision medicine is highlighted, wherein identifying specific molecular characteristics could guide personalised treatment strategies in the future.