Animal obesity models differ widely in type and extent of obesity. They are either based on environmental factors (e.g., high-fat diet-induced obesity), spontaneous mutants (i.e., ob/ob mice), genetically engineered animals (e.g., mice with melanocortin receptor subtype-4 gene disruption (knock-out), or mechanical intervention (e.g., chemical lesion of the ventromedial hypothalamus). This unit reviews available rodent models to study obesity and attempts to highlight the greatest utility for each model.