The production and deployment of polymorphonuclear neutrophils (PMNs) are under close regulation. PMNs interact through cytokines with a number of cell types, including macrophages, lymphocytes, and endothelial cells. PMNs are guided by bacterial products and cytokines to target sites, where microbes are recognized and killed. Killing occurs through oxygen-dependent and oxygen-independent mechanisms. The frequent and severe infections seen in patients with defects (either congenital or acquired) in PMN function demonstrate the importance of PMNs in host defense against infection. PMNs are potent inflammatory cells and can exacerbate disease states such as myocardial ischemia, gram-negative bacterial sepsis, and the adult respiratory distress syndrome.