Prostatitis syndromes are best understood in terms of the anatomy and physiology of the genitourinary tract. Infections of the prostate may occur despite numerous host defenses of the male lower urogenital tract. Histologic criteria for prostatitis correlate imperfectly with clinical and microbiologic findings. It is critical to distinguish patients with lower-urinary-tract complaints associated with bacteriuria from the larger number of patients without bacteriuria. Careful lower-urinary-tract localization studies may then be used to classify patients into four diagnostic groups: acute bacterial prostatitis, chronic bacterial prostatitis, nonbacterial prostatitis, and prostatodynia. Considerable progress has been made in understanding the pathophysiology and developing rational approaches for treatment of patients with acute and chronic bacterial prostatitis. Unfortunately, few reliable data are available on the etiology of nonbacterial prostatitis or prostatodynia. Thus, current therapy for most patients with prostatitis syndromes is unsatisfactory. Occasional patients develop granulomatous prostatitis, a characteristic histologic reaction of the prostate to a variety of insults. Treatment of granulomatous prostatitis depends on accurate etiologic diagnosis.