Abstract Chronic Prostatitis Syndrome (CPS) is a frequent disease. Before the PSA era, data from various sources showed that there were more outpatient visits for CPS than for BPH or prostate cancer. CPS has a very negative impact on the patients’ quality of life. However, this debilitating disease stimulates relatively little enthusiasm in the urological world. The classification proposed by the National Institutes of Health is widely adopted. The disease is described as “Chronic Prostatitis/Chronic Pelvic Pain Syndrome” and considers four categories. Category III includes Chronic Inflammatory Prostatitis and Prostatodynia. Universally accepted guidelines for diagnostic work-up are not available; so that the number of cases diagnosed as Chronic Bacterial Prostatitis differs significantly according to the investigations employed by various working groups. The role of Chlamydia and Ureaplasma as causative agents for CPS is still controversial, although many data are in favour of their responsibility in a relatively high number of patients with CPS. Diagnosis of CPS rests fundamentally on symptoms and on the physician's clinical judgement. CPS and interstitial cystitis may share common pathogenetic mechanisms. The first part of this review covers epidemiology, definition, classification, aetiology and physiopathology, natural history, complications, clinical manifestations and diagnostic work-up. Treatment will be dealt with in a subsequent review article.