Reactive oxygen species (ROS) play an essential role in regulating various physiological functions of living organisms. The intrinsic biochemical properties of ROS, which underlie the mechanisms necessary for the growth, fitness, or aging of living organisms, have been driving researchers to take full advantage of these active chemical species for contributing to medical advances. Thanks to the remarkable advances in nanotechnology, great varieties of nanomaterials with unique ROS-regulating properties have been explored to guide the temporospatial dynamic behaviors of ROS in biological milieu, which contributes to the emergence of a new-generation therapeutic methodology, i.e., nanomaterial-guided in vivo ROS evolution for therapy. The interdependent relationship between ROS and their corresponding chemistry, biology, and nanotherapy leads us to propose the concept of "ROS science", which is believed to be an emerging scientific discipline that studies the chemical mechanisms, biological effects, and nanotherapeutic applications of ROS. In this review, state-of-art studies concerning recent progresses on ROS-based nanotherapies have been summarized in detail, with an emphasis on underlying material chemistry of nanomaterials by which ROS are generated or scavenged for improved therapeutic outcomes. Furthermore, key scientific issues in the evolution of ROS-based cross-disciplinary fields have also been discussed, aiming to unlock the innate powers of ROS for optimized therapeutic efficacies. We expect that our demonstration on this evolving field will be beneficial to the further development of ROS-based fundamental researches and clinical applications.