Nitric oxide (NO) plays important roles as a signaling molecule and antimicrobial agent throughout biology. As with O2, many of its physiological effects are mediated by its interactions with hemoproteins. This chapter focuses on bacterial hemoproteins that function as NO sensors and NO scavengers. These functions are particularly important to pathogenic organisms as they provide a means to resist the NO produced by the host immune response. We begin by presenting structural data for NO sensor proteins that highlight mechanisms of signal transduction upon binding of NO to the heme iron. We then describe two mechanisms of NO detoxification, the NO dioxygenase and NO reductase reactions. The former is catalyzed by members of the globin family and produces nitrate (NO3
-), while the latter is catalyzed by heme/nonheme diiron NO reductases and produces nitrous oxide (N2O). A wealth of spectroscopic and rapid kinetics data on these proteins and synthetic or bioengineered analogues has begun to identify intermediates that provide insights into these physiologically important reactions.