Due to inadequate defence mechanisms, cirrhotic patients with ascites have an increased susceptibility to infections, the most frequent and the most severe one being spontaneous bacterial peritonitis (SBP). SBP diagnosis is based on testing of the ascitic fluid obtained by paracentesis. A polymorphonuclear cell count of more than 250 cells/mm3 of ascitic fluid is considered diagnostic and from cultures of ascitic fluid only one germ should be isolated. 60% of the SBP episodes are produced by gram negative enteric bacilli - E. coli and Klebsiella spp. being the most frequent isolated microorganisms. The most important pathogenic mechanism for SBP is bacterial translocation. In liver cirrhosis, three mechanisms are proposed for the pathogenesis of SBP: intestinal bacterial overgrowth, the alterations (structural and functional) of the intestinal mucosal barrier and the deficiencies of the local immune response. The most appropriate antibiotic treatment is a third generation cephalosporin (Cefotaxim or Ceftriaxon) which should be administrated for 5 days. With early start of the antibiotic treatment, the short-term prognosis of cirrhotic patients with SBP has improved significantly. Unfortunately, the long term prognosis remains extremely poor due to the severity of subjacent liver disease.