Neutrophils are considered to be central to the pathogenesis of most forms of acute lung injury (ALI). For the sake of clarity, neutrophil involvement in ALI can be conceptualized as consisting of sequential stages, beginning with their sequestration in the pulmonary microvasculature, followed by adhesion and activation, and culminating in the production of a microbicidal or "effector" response, such as the generation of reactive oxygen species or release of proteolytic enzymes. Great strides have been made in elucidating these various stages of neutrophil involvement. Recent studies have focused on the intracellular signaling pathways that govern neutrophil activation and have elucidated complex cascades of kinases and other intracellular signaling molecules that allow for amplication of the neutrophil response, yet simultaneously confer specificity of a response. We believe that the inflammatory response in ALI may initially be adaptive, such as the pivotal role played by neutrophils in a bacterial or fungal infection. Ultimately, it is the persistence or the dysregulation of neutrophil activation that may lead to ALI. An increased understanding of how neutrophils function will facilitate the design of therapeutic strategies that retain the beneficial aspects of the inflammatory response, while avoiding unnecessary tissue damage.