A biomarker is a biological molecule present in the blood, other body fluids or tissues, that indicates normal or abnormal process, condition or a disease. It can act as an alternative tool and is helpful in selecting accurate treatment in various diseases. Gastrointestinal cancer is a malignant condition affecting gastrointestinal tract and organs of the digestive system including the stomach, pancreas, esophagus, small and large intestine, biliary system, rectum and anus. In the case of esophageal cancer, methylation of DNA can be used as a biomarker for early detection because methylation patterns appear in the initial stages. Protein biomarkers are utilized as markers for gastric tumor for effective diagnosis, prognosis and checking for recurrence of cancer after treatment. Various biomarkers that are utilized in case of stomach cancer include antigens like CA12-5 and CA72-4 as well as alpha-fetoprotein. This book provides comprehensive insights on biomarkers. The topics covered herein deal with the core aspects of the detection of gastrointestinal cancers through biomarkers. This book will serve as a reference to a broad spectrum of readers.