Papillomas and papillary carcinomas differ in their 3 fundamental characteristics: the geometric properties of their fronds, the amount of their stroma, and the characteristics of their epithelium. Fibrosis at the edge of papillomas often entraps glands and creates the spurious impression of invasion. The proliferation of surface epithelial cells of papillomas does not give rise to unexpected diagnostic difficulties, but glandular proliferation within the stalks of papillomas often simulates the appearance of cribriform ductal carcinoma in situ. Needle biopsies of papillomas can deposit clusters of benign cells in a distribution that resembles an invasive carcinoma. Although papillomas overrun by ductal carcinoma in situ exhibit a papillary architecture, other features differentiate them from conventional papillary carcinomas. The presence of basal carcinoma cells with clear cytoplasm ("dimorphic" cells) and the formation of short stubby fronds sometimes cause pathologists to mistake papillary carcinomas for papillomas, and the bland cytologic characteristics of solid papillary carcinomas can lead to the same error. Conventional papillary carcinomas typically invade in a blunt manner. This phenomenon complicates the recognition of invasion by many papillary carcinomas and has given rise to controversy about the nature of the lesion classically known as "intracystic papillary carcinoma."