Increased clinical screening of men at risk for prostate cancer, and the realization of the benefits of performing multiple biopsies per prostate, have facilitated early detection of malignancy, while presenting the pathologist with a growing array of diagnostic findings. Interpretation of these findings requires discussion of the minimal criteria required for the diagnosis of cancer on needle biopsy within a wide spectrum of related histologic findings. This spectrum includes small acinar proliferations suspicious for but not diagnostic of cancer, benign mimics of cancer, the preinvasive entity of high-grade prostatic intraepithelial neoplasia, and various treatment effects. Clinical implications of these findings and other prognostic factors are detailed.