This book covers state-of-the-art topics covering evidences indicate that the gut microbiota can be harnessed for cancer prevention as well as to modulate the efficacy of chemotherapy and immunotherapy. This book includes: 1) characterization of the dysregulated microbiome in gastrointestinal cancers (Chapters 1-6); 2) the molecular mechanism of action of gut microbiota via microbial metabolites and direct interaction with host cancer or immune cells (Chapters 7-11); 3) key methodologies for studying the role of gut microbiota in cancers (Chapters 12-13); and 4) the potential application of gut microbes for the prevention and treatment of gastrointestinal cancers (Chapters 14-17). The contributors are international experts in molecular and cellular biology, microbiology, metabolomics, bioinformatics and physician scientists to provide in-depth reviews of this subject. This book provides a rich resource of information on this important topic for graduate students, basic researchers and physicians.