Increasing evidence suggests that gut microbiota is an environmental factor that plays a crucial role in obesity. However, the aetiology of obesity is rather complex and depends on different factors. Furthermore, there is a lack of consensus about the exact role that this microbial community plays in the host. The aim of this review is to present evidence about what has been characterized, compositionally and functionally, as obese gut microbiota. In addition, the different reasons explaining the so-far unclear role are discussed considering evidence from in vitro, animal and human studies.