Reactive oxygen species (ROS) levels are strongly linked to obesity and related diseases, particularly insulin resistance and type 2 diabetes. ROS, on the other hand, are thought to be important in intracellular signaling, which may be required for insulin sensitivity. Many theories have been developed to explain this apparent paradox, which has helped us understand this important small molecule better. While many intracellular ROS production sites have been identified, mitochondrial-generated ROS remains a major contributor in most cell types. Some factors described in this review influence mitochondrial ROS generation. Furthermore, this review has demonstrated the need for a new sensitive approach to quantifying ROS.