Reactive Oxygen Species (ROS) are a class of highly reactive Oxygen-containing ions and molecules that can be produced in cells by direct enzyme-mediated catalysis, as byproducts of mitochondrial respiration, and as secondary metabolites produced by the reaction of ROS with each other and other molecules in the cell. Identifying these short-lived molecules can be challenging due to their short-lived nature, as well as their similar chemistries and interactions with commonly used fluorescent ROS probes. In the immune response, ROS are produced in both fly and mammalian macrophages, where they play roles in both direct killing of engulfed microbes, as well as regulatory roles in macrophage activation and inflammation. We are dissecting the fly macrophage ROS response using a combination of genetic and biochemical tools, to better characterize ROS produced at different stages of the immune response, and to determine their distinct functions. We find that reactive Nitrogen species (RNS) produced by Nitric Oxide Synthase modulate the ROS response, and are responsible for some inflammatory processes previously ascribed to ROS.