Debates relevant to the etiology of weight gain or obesity, i.e., the dichotomous understandings about whether obesity arises from the genetic predisposition or from the environmental influences, has long existed. This is an important issue because it is related to the therapists's prejudice when treating patients with obesity. In this review, the authors first discuss the environmental and the genetic factors that cause the obesity, and in the latter part, the interactions between genetic and environmental factors will be discussed. This issue is considered and described especially in a conceptual aspect for the therapists ultimately to understand how the genetic and the environmental factors interact to arise obesity. Conclusively, obesity is best understood as a complex, multifactorial, and chronic disabled state, which cause an individual with genetic predispostion to obesity under the environmental influences. In future, in favor of the accumulated knowledge about the genetic and environmental impacts and their interactions in detail, we will be able to provide a client-specific management or prevention of obesity.