This book is designed to give a comprehensive overview of the field of Circulating Tumor Cells (CTCs), which are found in blood of patients with cancer. It includes descriptions of basic techniques used to capture and quantify CTCs, and methods employed to propagate them after capture for downstream study. An overview of various molecular approaches for CTC analysis and characterizations is provided, including their applicability as a "liquid biopsy" and their potential utility for predicting therapeutic response to treatments. A number of aspects involved in the metastatic spread of cancer are also discussed, as well as how quantitation and characteristics of CTCs relate to what is known about the metastatic cascade.