The histopathological diagnosis of malignancy has traditionally been said to be the gold standard by which all other methods of investigation and diagnosis are defined and assessed. Although this is still largely true, the other chapters in this book attest to the fact that it may not be true for much longer, or at least it may become less golden. More interestingly, it is probably already not true in the diagnosis of premalignant conditions. This is significant clinically because, as in all cancers, the earlier the diagnosis of prostate cancer, the better the chance of cure. However, histopathological diagnosis is still the most powerful tool in use today in patient management and, as such, it is appropriate to review the histopathological diagnosis of carcinoma, its methods, and protocols to define the arena in which other techniques are used and useful.