Neutrophils play a key role in the innate immune system, providing the first line of host defense. In addition to their ability to eliminate pathogens by phagocytosis and antimicrobial secretions, it has recently been shown that neutrophils can trap and kill microorganisms by the release of extracellular structures composed of DNA and antimicrobial proteins called neutrophil extracellular traps (NETs). Although physiological amounts of NETs are important as antimicrobial agents, high levels of NETs in circulation may result in severe tissue damage. Besides, the excessive generation of NETs or a disruption in their clearance mechanism might be associated with the development of certain autoimmune diseases. This review describes the structure, function and generation of NETs, and their possible implication in the initiation and/or progression of different diseases.