Summary: Reactive oxygen species (ROS) and reactive nitrogen oxide species (RNOS), including nitric oxide, are produced in cells by a variety of enzymatic and non-enzymatic mechanisms. At high levels, both types of oxidants are used to kill ingested organisms within phagocytes. At low levels, RNOS may diffuse outside cells where they impact the vasculature and nervous system. Recent evidence suggests that low levels of ROS produced within cells are involved in cell signaling. Along with these physiological roles, many pathological conditions exist where detrimental high-level ROS and RNOS are produced. Many situations in which ROS/RNOS are associated also involve mast cell activation. In innate immunity, such mast cells are involved in the immune response toward pathogens. In acquired immunity, activation of mast cells by cross-linking of receptor-bound immunoglobulin E causes the release of mediators involved in the allergic inflammatory response. In this review, we describe the principle pathways for ROS and RNOS generation by cells and discuss the existence of such pathways in mast cells. In addition, we examine the evidence for a functional role for ROS and RNOS in mast cell secretory responses and discuss evidence for a direct relationship between ROS, RNOS, and mast cells in mast cell-dependent inflammatory conditions.