The key aim of the proposed chapter is to provide readers a brief description for the most important parts of the field of circulating tumor cells (CTCs): the core techniques, including negative and positive selection-based CTC isolation, and the differences between them. Most importantly, we will also review the clinical applications and important findings in clinical trials. The evidence-based review will not only help clinicians use CTCs to predict recurrence and foresee the disease-related outcomes but also to inspire the researchers in this field to conduct further investigations.