Prostate cancer offers a wide spectrum ranging from clinically insignificant to aggressive and fatal disease. The Gleason grade is a powerful prognostic indicator and does influence treatment decision. Educational programs and websites are currently available to improve reproducibility and reliability of Gleason grading. A major problem in reporting of Gleason grading in needle biopsy is undergrading. The Gleason grade 3 is the lowest grade which can be assigned reliably in needle biopsies. The major prognostic shift is between Gleason grades 3 und 4 which is characterized by fusion of the glandular formations. Reporting the proportion of Gleason grades 4 and 5 in needle biopsies may be critical in terms of treatment decisions. The present review deals with diagnostic criteria of the Gleason grades and its clinical implications.