Abstract: In recent years, advances in our understanding of the molecular biology of prostate cancer have spawned optimism that new biomarkers will allow greater sensitivity and specificity in diagnosis as well as more accurate prediction of outcome after treatment for the individual patient. For at-risk patients with negative biopsy, multiple new markers are available, including mitochondrial DNA deletion testing (tissue), PCA3 (urine), and sarcosine (tissue or blood). For patients with diagnostically challenging prostate biopsies, multiple stain combinations (multiplex immunohistochemistry) such as quadruple staining with racemase, high-molecular-weight cytokeratin, p63, and c-myc are commonly used to confirm the diagnosis of prostate cancer; the recent addition of c-myc to create a quadruple stain provides additional diagnostic support for cancer, recognizing that there may be heterogeneity or complete lack of staining for some of the other markers. These stain combinations must, of necessity, be used together on 1 slide owing to common loss of the focus of concern on deeper sections. TMPRSS2:ERG is a useful diagnostic adjunct for select cases, with the recent introduction of antibody staining. Prostate-specific antigen, prostatic acid phosphatase, and prostate-specific membrane antigen are useful for confirming prostatic origin of cancers, as well as for predicting outcomes for select patient group. Other biomarkers are being actively investigated as companion diagnostics to predict response to treatment. This report describes the current state of immunohistochemical stains for the diagnosis of prostatic intraepithelial neoplasia, atypical small acinar proliferation, and prostate cancer.