Most bacterial infections in cirrhotic patients are hospital-acquired. Urinary tract infections, spontaneous bacterial peritonitis (SBP), respiratory tract infections, and bacteremia are the most frequent bacterial infectious complications seen in cirrhotic patients. SBP is the most characteristic infectious complication of cirrhotic patients, and it is defined as the infection of a previously sterile ascitic fluid, with no apparent intra-abdominal source of infection. The incidence of SBP in cirrhotic patients admitted to hospital with ascites has been estimated to range between 7 and 23%. The diagnosis is established on the basis of clinical signs and symptoms and/or a polymorphonuclear cell count in ascitic fluid higher than 250/mm3. This diagnosis is confirmed by a positive culture in approximately 70% of the cases. The remaining 30% are considered culture-negative SBP but are empirically treated with antibiotics because severe peritonitis and death may follow if these patients are not treated. Early diagnosis, the routine use of diagnostic paracentesis in patients admitted to hospital with ascites, and, especially, the use of adequate antibiotics are very important tools in the treatment of SBP. Third-generation cephalosporins are the first-choice antibiotic treatment in SBP, although selected patients with SBP, those with normal renal function and without hepatic encephalopathy, shock, or gastrointestinal bleeding, may be treated with oral quinolones. Selective intestinal decontamination with norfloxacin is safe and useful in the primary and secondary prophylaxis of SBP, although the incidence of quinolone-resistant organisms is increasing and this may be a problem in the future.