Pancreatic intraepithelial neoplasias (PanINs) are microscopic lesions of the pancreas. Traditionally viewed as a benign metaplasia of small ducts, evidence suggests that PanINs are neoplastic and that some PanINs progress to invasive ductal adenocarcinoma. The primary diagnostic challenge is distinguishing PanINs from other lesions, including invasive ductal adenocarcinoma, intraductal papillary mucinous neoplasm, and cancerization of benign ducts. PanINs are the most common of the pancreatic cancer precursor lesions, yet they remain poorly understood and are so small that they are almost clinically undetectable. Further study is required to define the role of PanINs in the carcinogenesis and early detection of pancreatic cancer.