Oxidants are produced as a by-product of aerobic metabolism, and organisms ranging from prokaryotes to mammals have evolved with an elaborate and redundant complement of antioxidant defenses to confer protection against oxidative insults. Compelling data now exist demonstrating that oxidants are used in physiological settings as signaling molecules with important regulatory functions controlling cell division, migration, contraction, and mediator production. These physiological functions are carried out in an exquisitely regulated and compartmentalized manner by mild oxidants, through subtle oxidative events that involve targeted amino acids in proteins. The precise understanding of the physiological relevance of redox signal transduction has been hampered by the lack of specificity of reagents and the need for chemical derivatization to visualize reversible oxidations. In addition, it is difficult to measure these subtle oxidation events in vivo. This article reviews some of the recent findings that illuminate the significance of redox signaling and exciting future perspectives. We also attempt to highlight some of the current pitfalls and the approaches needed to advance this important area of biochemical and biomedical research.