Prostatic intraepithelial neoplasia (PIN) represents the putative precancerous end of the morphologic continuum of cellular proliferations within prostatic ducts, ductules and acini. Two grades of PIN are identified (low grade and high grade), and high grade PIN is considered to be a precursor to invasive carcinoma. The continuum which culminates in high grade PIN and early invasive cancer is characterized by basal cell layer disruption, basement membrane disruption, progressive loss of secretory differentiation markers, increasing nuclear and nucleolar abnormalities, increasing proliferative potential, and increasing variation in DNA content (aneuploidy). Clinical studies suggest that PIN predates carcinoma by ten years or more, with low grade PIN first emerging in men in the third decade of life. The clinical importance of recognizing PIN is based on its strong association with carcinoma; its identification in biopsy specimens of the prostate warrants further search for concurrent invasive carcinoma.