Flow cytometry represents an interesting methodological approach to human neutrophil function assessment and to the study of neutrophil biology. Several aspects of neutrophil activation can be evaluated by means of such technology: phagocytosis, respiratory burst, surface markers, modifications in intracellular pH, actin polymerization, membrane potential, aggregation. Neutrophil flow cytometric evaluation is suitable for whole blood evaluation and has been shown to be very specific and sensitive. In particular, whole blood methods have been developed in order to investigate phagocytosis, hydrogen peroxide production, and surface markers expression (integrins, receptors for the Fc fragment of IgG, selectins, etc.). Other aspects of neutrophil activation have to be investigated by using purified neutrophils.