Cancers of the gastrointestinal tract, including the liver, bile ducts, and pancreas, constitute the largest group of malignant tumors. Colorectal cancer is one of the most common neoplastic diseases in Western countries and one of the leading causes of cancer-related deaths. Inactivation of the adenomatous polyposis coli (APC) tumor-suppressor gene during early adenoma formation is thought to be the first genetic event in the process of colorectal carcinogenesis followed by mutations in oncogenes like K-Ras and tumor-suppressor genes like p53. Identification of the interaction of APC with the proto-oncogene β-catenin has linked colorectal carcinogenesis to the Wnt-signal transduction pathway. The main function of APC is thought to be the regulation of free β-catenin in concert with the glycogen synthase kinase 3β (GSK-3β) and Axin proteins. Loss of APC function, inactivation of Axin or activating β-catenin mutations result in the cellular accumulation of β-catenin. Upon translocation to the nucleus β-catenin serves as an activator of T-cell factor (Tcf)-dependent transcription leading to an increased expression of several specific target genes including c-Myc, cyclin D1, MMP-7, and ITF-2. While APC mutations are almost exclusively found in colorectal cancers, deregulation of Wnt/β-catenin/Tcf signaling is also common in other gastrointestinal and extra-gastrointestinal human cancers. In a fraction of hepatocellular carcinomas the Wnt pathway is deregulated by inactivation of Axin or stabilizing mutations of β-catenin. The majority of hepatoblastomas and a group of gastric cancers also carry β-catenin mutations. Clearly, this pathway harbors great potential for future applications in cancer diagnostics, staging, and therapy.