Neutrophils are polynuclear cells essential to innate immunity. They are the first cells to migrate from the blood to the inflammatory site where they kill pathogens and secrete various mediators that regulate innate and adaptive immunity. Functional steps required for their microbicidal activity include: transendothelial migration, migration towards the invading pathogens, and then recognition, adhesion, engulfment, and killing of the target. Primary deficiencies of these stages are expressed by repeated and/or severe bacterial and fungal infections. These deficiencies include granule abnormalities and leukocyte adhesion deficiencies Type I and II, defective pathogen recognition and the defective oxidative burst that characterizes chronic granulomatous disease.