Reactive oxygen species (ROS) are a family of molecules that are continuously 
generated, transformed and consumed in all living organisms as a consequence of 
aerobic life. The traditional view of these reactive oxygen metabolites is one 
of oxidative stress and damage that leads to decline of tissue and organ systems 
in aging and disease. However, emerging data show that ROS produced in certain 
situations can also contribute to physiology and increased fitness. This 
Perspective provides a focused discussion on what factors lead ROS molecules to 
become signal and/or stress agents, highlighting how increasing knowledge of the 
underlying chemistry of ROS can lead to advances in understanding their 
disparate contributions to biology. An important facet of this emerging area at 
the chemistry-biology interface is the development of new tools to study these 
small molecules and their reactivity in complex biological systems.