Hairpin vortices are found on top of turbulent bulges of the turbulent wall, wrapping around the turbulent wall in hairpin shaped loops, where the name originates. The hairpin-shaped vortices are believed to be one of the most important and elementary sustained flow patterns in turbulent boundary layers. Hairpins are perhaps the simplest structures, and models that represent large scale turbulent boundary layers are often constructed by breaking down individual hairpin vortices, which could explain most of the features of wall turbulence. Although hairpin vortices form the basis of simple conceptual models of flow near a wall, actual turbulent flows may contain a hierarchy of competing vortices, each with their own degree of asymmetry and disturbances.