The sharp rise in prevalence of obesity in recent decades has been suggestively labeled as an "epidemic," and the lack of fully explanatory causal factors has challenged existing understandings of obesity's etiology from a purely energetic standpoint. Much recent attention has been focused on the microbial members of the human gut for insights into their role in potentially causing or promoting obesity. The human gut is home to trillions of microbes, among which hundreds of distinct species of bacteria interact to form the human gut microbiome, and numerous studies in humans and animal models have linked shifts in the gut microbiome to obesity. In this review we explore contemporary understandings of the relationship between obesity and the microbiome from a high-level ecological and functional perspective, along with a survey of recently proposed interventions. We highlight areas of consensus and areas for further study in the field.