Energy homeostasis is regulated by twin factors, energy intake and energy expenditure. Obesity arises when these two factors are out of balance. Recently, the microflora residing in the human gut has been found to be one of the influential factors disturbing energy balance. Recent interest in this field has led to use of the term “gut microbiome” to describe the genomes of trillions of microbes residing in the gut. Metagenomic studies have shown that the human gut microbiome facilitates fermentation of indigestible carbohydrates to short-chain fatty acids that provide excess energy to the body, thus contributing to the obese phenotype. Alteration in the ratio of Bacteroidetes and Firmicutes drives a change in fermentation patterns that could explain weight gain. Therefore, changes in the gut microbiome (induced by antibiotics or dietary supplements) may be helpful in curbing the obesity pandemic. This review provides information on the expansive role the gut microbiome is believed to play in obesity and other related metabolic disorders.