Support is requested for a series of annual Keystone Symposia meetings on the topic of Obesity. The first meeting in the series will be held in 2015 and is entitled Obesity and the Metabolic Syndrome: Mitochondria and Energy Expenditure, and is being organized by Johan Auwerx, Eleftheria Maratos-Flier and Thomas Langer. The meeting will be held in Whistler, British Columbia, Canada from March 22-27, 2015. A medical challenge of the 21st century is the coupling of an unprecedented increase in life expectancy with a large increase in incidence and prevalence of chronic multifactorial diseases. Classic disease examples include disorders of metabolism such as obesity and type 2 diabetes mellitus, which are often associated with co- morbidities including non-alcoholic liver disease and coronary, peripheral and central vascular disease. Collectively the spectrum of disorders is often referred to as the metabolic syndrome. Over the past decade, it has become evident that shifts in cellular metabolism are often linked to changes in the number, morphology, and function of mitochondria, and that these cellular changes underlie systemic metabolic abnormalities that characterize the many clinical features of the metabolic syndrome. Keystone Symposia's 2015 meeting on Obesity and the Metabolic Syndrome will discuss novel findings that link mitochondrial function in peripheral tissues, such as liver, muscle and gut, with the systemic abnormalities of the metabolic syndrome. The potential for identifying new therapeutics that are based on modifying mitochondrial function will be reviewed. As mitochondrial function is now acknowledged to also control metabolic homeostasis on an organismal level in addition to controlling energy homeostasis at the cellular level this meeting will create a truly interdisciplinary environment that brings together basic scientists in the fields of bioenergetics, mitochondrial and cell biology, physiology and clinicians with an interest in metabolic disease, to foster innovative ideas in this area. Opportunities for interdisciplinary interactions will be significantly enhanced by the concurrent meeting on Liver Metabolism and Nonalcoholic Fatty Liver Disease, which will share a plenary session with this meeting.