Inflammatory bowel syndrome (IBS) mainly includes ulcerative colitis (UC) and Crohn's disease (CD). UC and CD are chronic and recurrent conditions, with a tendency to exacerbations and remissions. The incidence of diseases worldwide has increased over the last years. Although the etiology of inflammatory bowel syndrome has been studied intensively it still remains unclear. The development and persistence of inflammation is an effect of numerous factors: proinflammatory (aggressive), regulating bowel mucosa homeostasis and protective factors. Proinflammatory factors include intestinal bacteria, bile acids, digestive enzymes, lipopolysaccharides and peptidoglycans. Protective mechanisms are impermeability of mucosa barrier, presence of intestinal mucus, activity of secretive immunoglobulins, some prostaglandins and interleukins, glutamine, somatostatin, cortisol and short-chain fatty acids. Factors modifying intestinal mucosa homeostasis consist of genetically determined immunoregulators and activity of intestinal mucosa barrier and some environmental factors (diet, smoking, infections, stress, antibiotics and others). Environmental factors are jointly responsible for IBS occurrence in case of genetically determined dysregulation leading to proinflammatory cytokines overproduction or disturbances in synthesis of cytokines regulating intestinal mucosa homeostasis.