Columnar cell lesions of the breast represent a spectrum of lesions which have in common the presence of columnar epithelial cells lining variably dilated terminal duct lobular units, ranging from those that show little or no cytologic or architectural atypia to those that show sufficient cytologic and architectural features to warrant a diagnosis of atypical ductal hyperplasia or ductal carcinoma in situ. Recent studies have begun to provide insights into the biological nature and clinical significance of these lesions. In this article, we review the current state of knowledge and propose a simplified scheme for their classification.