Serrated polyps of the colorectum form a group of related lesions which include aberrant crypt foci (ACF), conventional hyperplastic polyps, mixed (admixed) polyps, serrated adenomas and sessile serrated adenomas. In recent years the molecular differences between these morphologically similar lesions have been highlighted, and their differing biological potential has been realised. In particular, the sessile serrated adenoma has become recognised as the precursor lesion to a group of sporadic colorectal carcinomas characterised by morphological and molecular features distinct from conventional adenomas. These recent findings have challenged the long held paradigm that all colorectal carcinomas arise via the traditional adenoma-carcinoma sequence. In addition, they present a major challenge for the early detection and management of colorectal cancer, which is no longer regarded as a homogeneous entity. Serrated polyps of the colorectum form a group of related lesions which include aberrant crypt foci (ACF), conventional hyperplastic polyps, mixed (admixed) polyps, serrated adenomas and sessile serrated adenomas. In recent years the molecular differences between these morphologically similar lesions have been highlighted, and their differing biological potential has been realized. In particular, the sessile serrated adenoma has been recognized as the precursor lesion to a group of sporadic colorectal carcinomas characterised by morphological and molecular features distinct from conventional adenomas. These recent findings have challenged the long held paradigm that all colorectal carcinomas arise via the traditional adenoma-carcinoma sequence.