In addition to the known utility of the copper ion in both stoichiometric and catalytic oxidative transformations [1-4], this metal is also well established as a required trace element having a crucial role in proteins involved in electron transfer and dioxygen (O2) processing [5-14]. Copper enzyme active sites possessing a diversity of coordination structures are involved in O2 transport, mixed function oxidases (monooxygenases), dioxygen transferases (dioxygenases), and electron transfer oxidases (oxidases). The interaction of Cu(I) with dioxygen is an essential component of both chemical and biological processes, prompting considerable research efforts on fundamental aspects of O2 reactivity with wellcharacterized low-molecular-weight copper complexes. It is the purpose of this chapter to summarize what is known about copper/O2 interactions in proteins and in chemical systems. We will emphasize biomimetic studies, focusing on advances in the reversible binding of dioxygen to Cu(I) complexes and in the related ‘‘activation’’ of O2, where monooxygenation of substrates occurs.