It is now well established that a healthy gut flora is largely responsible for the overall health of the host, while a perturbation in gut microbial communities can contribute to disease susceptibility. Obesity is a complex process involving genetic and environmental factors with an epidemiological burden that makes it a major public health issue. Studies of germ-free or gnotobiotic mice provided evidence that the diversity, as well as the presence and relative proportion of different microbes in the gut play active roles in energy homeostasis. Similarly, human studies showed that both the diversity of the microbiota and the Bacteroidetes/Firmicutes ratio are decreased in obese individuals. The 'obese microbiota' seems to be able to increase dietary energy harvest and favor weight gain and fat deposition. Although research in this field has just started and many of the available data are still conflicting, the results are providing exciting perspectives, and gut microbiota manipulation has already become a new target for both prevention and treatment of obesity.