Reactive oxygen species (ROS) play an essential role in different biological functions, including physiological regulatory mechanisms to damaging alterations involved in the pathogenesis of an increasing number of human diseases. ROS are metabolic products from numerous cells, but two vital cellular organelles are intimately involved in their production and metabolism, i.e. the endoplasmic reticulum and the mitochondria. Excess cellular levels of ROS may cause damage to proteins, nucleic acids, lipids, membranes, and organelles, which may lead to the activation of cell death processes such as apoptosis. Apoptosis is a highly regulated process that is essential for the development and survival of multicellular organisms. These organisms often need to discard superfluous or potentially harmful cells, have accumulated mutations, or become infected by pathogens. Apoptosis features a characteristic set of morphological and biochemical features whereby cells undergo a cascade of self-destruction. Thus, proper regulation of apoptosis is essential for maintaining normal cellular homeostasis. Mitochondrial ROS (mtROS) play a central role in cell signaling and regulating the main pathways of apoptosis, mediated by mitochondria, death receptors, and the endoplasmic reticulum (ER) stress. This book focuses on the current understanding of the role of ROS in each of these three main pathways of apoptosis related to different human diseases, with particular emphasis on Metabolic, Inflammatory, Neurodegenerative, and Neoplastic diseases. It's promise and limitations are also discussed by targeting ROS with different antioxidants in preventing non-communicable diseases. The role of mtROS in the complex interplay and crosstalk between these different signaling pathways of apoptosis related to different human diseases remains to be unraveled further in future research.