Obesity is the outcome of an extreme difference between the energy intake and the energy expended, which leads to severe weight gain. The two main factors of obesity are environmental influences and genetics, but the extent of the genetic contribution to obesity continues to be unknown. Multiple studies using different tools have been used to support the significance of the genetic influence on obesity, such as twin and adoption studies, race/ethnicity, and mouse models. The use of mouse models has allowed for the greatest expansion of knowledge on the genetics behind obesity, and advancements continue to be made to this tool. There are many possibilities on how genetic variations could affect the development of obesity including eating behavior, metabolism and digestion, energy intake, and adiposity. The various tools that have been used in studies on obesity and the vast number of factors that are dictated by genes prove that the influence of genetics on obesity is substantial.