Reactive oxygen species (ROS) production and oxidative stress (OS) in adipose tissue are associated with obesity and insulin resistance (IR). The nature of this relationship i.e., cause and effect or consequence has not been clearly determined. We provide evidence that elevated mitochondrial ROS generated by adipocytes from mice with diet-induced obesity (DIO) represents an adaptive mechanism that precipitates fatty acid oxidation, mitochondrial biogenesis, and mitochondrial uncoupling in an effort to defend against weight gain. Consistent with that, mice with adipocytespecific deletion of manganese superoxide dismutase (MnSOD) exhibit increased adipocyte superoxide generation and are protected from weight gain and insulin resistance which otherwise develops in wild-type (WT) mice that consume an obesogenic diet. The defense mechanism displayed by MnSOD-deficiency in fat cells appears to be mediated by a dual effect of ROS on inefficient substrate oxidation through uncoupling of oxidative phosphorylation and enhanced mitochondrial biogenesis. The aim of this commentary is to summarize and contextualize additional evidence supporting the importance of mitochondrial ROS in the regulation of mitochondrial biogenesis and the modulation of uncoupling protein 1 (UCP1) expression and activation in both white and brown adipocytes. Mitochondria are the major sites of substrate oxidation in mammalian cells and a major source of ROS.1-3 Mitochondria can generate superoxide and hydrogen peroxide (H 2 O 2 ) from at least 11 different oxidoreductases associated with substrate catabolism and the electron transport chain (ETC).4 ROS generation by mitochondria is a tightly controlled process i.e., low levels for cellular signaling are allowed whereas high levels that can damage the cellular milieu are mitigated by endogenous antioxidant enzymes. Importantly, there is substantial uncertainly regarding the different mechanisms controlling increased mitochondrial ROS generation in vivo, as well as the type of ROS species that are relevant for signaling in different aspects of physiology. Superoxide is the short-lived proximal ROS generated by mitochondrial oxidoreductases, and is rapidly converted to H 2 O 2 by manganese superoxide dismutase (MnSOD) in the mitochondrial matrix.5 Superoxide that is generated and released in the mitochondrial intermembrane space by complex III and the enzyme p66Shc is converted to H 2 O 2 by the Cu,ZnSOD enzyme.6-8 Matrix H 2 O 2 is degraded by a glutathione-dependent system catalyzed by glutathione peroxidases and the peroredoxin-thioredoxin system, 9 whereas cytosolic degradation of H 2 O 2 is mainly controlled by catalase, cytosolic peroxidases, and peroxiredoxins. 10Distinct from other antioxidant enzymes, MnSOD is important due to its localization in the mitochondrial matrix, and to its high affinity for superoxide in that compartment. 5 The physiologic relevance of MnSOD was demonstrated by the robust phenotype of mice lacking the in vitro Sod2 gene (encoding MnSOD). In this regard, Sod...