A diagnosis of localized prostate cancer poses an array of treatment decisions for patients and treating physicians. Prostate cancer is notable for the remarkable diversity in clinical outcomes that range from an indolent course, with no impact on the patient if left untreated, to rapid progression to metastatic disease and cancer-specific death. Consequently, treatment options also span conservative approaches, such as active surveillance to radical curative therapy that involves prostatectomy or radiotherapy. Unfortunately, these interventions have adverse effects that can substantially affect quality of life. Thus, a major objective involves stratifying patients based on individual risk for aggressive disease behavior and basing treatment recommendations accordingly. Traditionally, risk stratification uses readily available clinical parameters that include the level of serum prostate-specific antigen, clinical stage, and the histological Gleason grade assessed from prostate biopsy. While clearly useful as guidelines for estimating risks of populations, the resulting classifications of low, intermediate, and high risk, as well as subdivisions within them, are far from perfect in determining what the precise trajectory will be for an individual patient.