There are trillions of microorganisms in the human intestine collectively called gut microbiota. Obesity may be affected by the gut microbiota through energy harvesting and fat storage by the bacteria. Small intestinal bacterial overgrowth is also responsible for endotoxemia, systemic inflammation, and its consequences including obesity and nonalcoholic fatty liver disease (NAFLD). Relationship between gut microbiota and NAFLD is also dependent on altered choline and bile acid metabolism and endogenous alcohol production by gut bacteria. Further evidence linking gut microbiota with obesity and NAFLD comes from studies showing usefulness of probiotics in animals and patients with NAFLD. This article reviews the relationship among gut microbiota, obesity, and NAFLD.