Polymorphonuclear neutrophils (PMNs) are the most abundant leukocytes in the blood and are considered as the irst line of innate immune defence against infectious diseases. However, PMN cells have a crucial function in both innate and adaptive immune responses.Neutrophils have several mechanisms to control pathogens, and one of them is their capability to form neutrophil extracellular traps (NETs) that may control infection. NETs have the capacity to trap microorganisms, kill them, or avoid their dissemination. The aim of this chapter is to provide a comprehensive review on NETs, the cells that produce them, and some of the mechanisms involved in their formation, their role in the immune response, and the pros and cons of NETs, focusing mainly on infectious diseases.