The necessity of early detection of prostate cancer renewed interest regarding putative premalignant lesions in the tumorigenesis of the prostate. Prostatic intraepithelial neoplasia (PIN) is one potential precursor for prostatic adenocarcinoma. The term PIN has been adopted to replace a wide range of synonyms in the literature that describe potential precursors. PIN is an intraluminal proliferation of the secretory cells lining architecturally benign prostatic ducts and acini that exhibit cytologic atypia. In this review, we discuss the histologic features, the differential diagnosis, the evidence that PIN is a precursor of prostatic carcinoma, and the clinical significance of PIN.