This article reviews the Gleason grading system and compares it to some of the other commonly used grading systems. Issues concerning both intra-and interobserver reproducibility of the Gleason grading system are presented. This article provides an in-depth analysis of the correlation between biopsy and radical prostatectomy Gleason score. New data from a study performed at Johns Hopkins Hospital provides reasons for discrepant grading between biopsy and a resection specimen. Finally, the prognostic capabilities of the Gleason grading system as it relates to radical prostatectomy, needle biopsy, and transurethral resection are discussed.