A review. Calorie restriction (CR), the only method that extends maximum life span in organisms ranging from yeast to mammals, was found to promote mitochondrial biogenesis by inducing endothelial nitric oxide synthase expression. Reactive oxygen species (ROS) signaling is very important to applications of photodynamic therapy (PDT) as well. It was pointed out that understanding the mechanisms of ROS signaling network in PDT might provide means to modulate the PDT effects at the mol. level and potentiate its antitumor effectiveness. Recently, the development and use of a technique mapping ROS at organismic level, in vivo ESR (EPR) for strictly exptl. applications in animals, was very successful, and the results have even made possible potential clin. applications. At organismic level mapping ROS signaling network in animals and human, EPR could enable more effective applications of CR and PDT. Cooperation between CR, PDT and ERP would lead a new and common road map for these three biomedical fields.