This book provides a wealth of images and extensive information on circulating tumor cells (CTCs) and other cells usually observed in blood from patients with cancer, such as giant macrophages, and circulating tumor microemboli (CTM).  The field of  liquid biopsy  began with the analysis of CTCs in the early 2000s. In the beginning, molecular techniques were developed to detect these cells in the blood. However, it has since become clear that CTCs initially require a cytopathological analysis to be detected without false positive and negative results. After detection, molecular analysis can be subsequently performed.  Cancer is an important cause of mortality, especially when detected in late stages. Even with all the advances that have been made in its treatment, cancer is still challenging, as many patients do not respond to any therapy. Many health agencies have considered early diagnosis as a feasible tool.