Nitroxyl (HNO) is the one-electron reduced and protonated congener of NO. It has been reported that HNO has numerous important and therapeutically useful biological properties. Although it has not been demonstrated that HNO is endogenously generated, developing HNO-donors as pharmacological agents has become an area of significant interest. Similar to NO (although chemically distinct), two primary sites of HNO interaction with biological systems are thiols/thiol proteins and metals/metalloproteins. It is likely that the chemistry of HNO with these types of biological targets is responsible for its unique and potentially useful biological properties. Interestingly, other established small-molecule signaling agents such as hydrogen peroxide share a target profile with HNO. Thus, HNO and H 2 O 2 may be intimately linked in their signaling functions. This chemistry and relationship will be discussed.