Obesity has become one of modern society's most serious health problems. Studies from the last 30 years revealed a direct relationship between imbalanced energy intake and increased healthcare costs related to the treatment or management of obesity. Recent research has highlighted significant effects of gut microbial composition on obesity. We aimed to report the current knowledge on the definition, composition, and functions of intestinal microbiota. We have performed an extensive review of the literature searching for the following key words: metabolism, gut microbiota, dysbiosis, and obesity. There is evidence that an association between intestinal microbiota and obesity exists at any age. There are complex genetic, metabolic, and inflammatory mechanisms involved in the pathogenesis of obesity. Revision of indications for use of probiotics, prebiotics, and antibiotics in obese patients should be considered. Microbial composition of the gut may be an important factor involved in the development of obesity. Changes in the gut microbiota may result in changes in human metabolism and weight loss.