This book explores the relationship between the gut microbiome and cancer, illuminating various facets from fundamental roles to personalized therapies. It provides a comprehensive overview of the impact of the gut microbiome on cancer development, offering potential for innovative diagnostic and treatment approaches. The book discusses the role of dysbiosis in cancer development, the influence of the microbiome on treatment responses, and strategies to modulate the microbiome for enhanced therapies. It also reviews the influence of the gut microbiome on immunotherapy resistance and chemoresistance in cancer patients. Additionally, the book presents gut microbiome biomarkers for cancer diagnosis and prognosis, the implication of the gut-brain axis on cancer development and progression, and the impact of diet and lifestyle on the microbiome. Towards the end, the book investigates the role of the gut microbiome in pediatric cancer and provides an outlook on the future challenges, technological advancements, and ethical considerations of gut microbiome research in the context of personalized cancer therapies. This book is intended for cancer researchers, medical oncologists, clinicians, pharmacologists, translational investigators, and medical students.