Most pancreatic cancers are not diagnosed until after the cancer has spread to other organs and is no longer curable. As a result, the death rate for pancreatic cancer in this country (34,290/year) is approximately equal to the incidence rate (37,680/year) (1). By contrast, many patients survive the diagnosis of breast cancer, and half of the decline in breast cancer mortality in the last quarter of a century has come from improved early detection (2). We believe that early detection of preinvasive lesions is the greatest hope for curing pancreatic neoplasia. This chapter discusses pancreatic intraepithelial neoplasia (PanIN), the most common precursor lesion in the pancreas (3).