The terms ''pre-cancer'', 'precursor lesions', 'premalignant','intra epithelial neoplasia' and 'potentially malignant' have been used in the international literature to broadly describe clinical presentations that may have a potential to become cancer. Precancers are lesions that precede the appearance of invasive cancers.The successful prevention or treatment of precancers has the potential to eliminate mortality due to cancer. This book provides the concepts of precancer with a note on its molecular pathogenesis as precancer has some, but not all, of the molecular and phenotypic properties that characterise the cancer.