Oxidative stress is recognized as free radical dyshomeostasis, which has damaging effects on proteins, lipids and DNA. However, during cell differentiation and proliferation and other normal physiological processes, free radicals play a pivotal role in message transmission and are considered important messengers. Organisms maintain free radical homeostasis through a sophisticated regulatory system in which these "2faced" molecules play appropriate roles under physiological and pathological conditions. Reactive oxygen species (ROS), including a large number of free radicals, act as redox signalling molecules in essential cellular signalling pathways, including cell differentiation and proliferation. However, excessive ROS levels can induce oxidative stress, which is an important risk factor for diabetes, cancer and cardiovascular disease. An overall comprehensive understanding of ROS is beneficial for understanding the pathogenesis of certain diseases and finding new therapeutic treatments.This review primarily focuses on ROS cellular localization, sources, chemistry and molecular targets to determine how to distinguish between the roles of ROS as messengers and in oxidative stress. K E Y W O R D Shomeostasis, oxidative stress, reactive oxygen, redox signalling, signalling pathways 2 of 16 | HUANG ANd LI for the development of new therapeutic strategies for cancer and cardiovascular disease. | SOURCES AND PHYSICOCHEMICAL PROPERTIES OF ROS | ROS sourcessecond messengers, what are the corresponding acceptors for these special ROS? To better recognize ROS, a new tool that can monitor changes in ROS levels and position in real time must be developed. Determining how to utilize the characteristics of ROS to treat diseases, such as cardiovascular disease, diabetes, autoimmune diseases, cancer and Alzheimer's diseases, is worth further exploration.