Accumulation of epigenomic aberrations plays a critical role in tumorigenesis, both in tumor initiation and progression. Development of epigenomic markers can therefore be useful in determination of tumor risk of apparently normal cases, or in classifying tumor cases in relation to etiology, prognosis or therapy response. Loss of imprinting of IGF2 is an epigenetic aberration that exists in normal tissues and modifies tumor risk, and is a possible therapy target for tumor risk management. Thanks to development of quantitative and comprehensive analysis tools for DNA methylation, it becomes possible to epigenotype tumors, i.e., classification of tumors using epigenomic information, by unsupervised hierarchical clustering. Gastric cancer is epigenotyped in relation to etiology such as EB virus infection, and colorectal cancer is epigenotyped in relation to oncogene mutation statuses, suggesting distinct molecular mechanisms in colorectal tumorigenesis.