Most cases of carcinoma involving the prostate gland show characteristic acinar histologic features. The term variant is used to describe a distinctly different histomorphologic phenotype of a certain type of neoplasm. The recognition of histologic variants of prostate carcinoma is important because some types are associated with a different clinical outcome and might have a different therapeutic approach, and because awareness of the unusual pattern might be critical in avoiding diagnostic misinterpretations. In this article, we review the subject of histologic variants of prostatic adenocarcinoma with a focus on histologic criteria and clinical significance. We discuss small cell carcinoma, ductal (endometroid) carcinoma, sarcomatoid carcinoma, signet ring cell carcinoma, mucinous carcinoma, lymphoepithelioma-like carcinoma, adenosquamous carcinoma of the prostate, squamous carcinoma, adenoid cystic carcinoma, and transitional cell carcinoma involving the prostate.