  Solar jets are observed as collimated plasma beams over a large range of
temperatures and wavelengths. They have been observed in Halpha and optical
lines for more than 50 years and called surges. The term "jet" comes from X-ray
observations after the launch of the Yohkoh satellite in 1991. They are the
means of transporting energy through the heliosphere and participate to the
corona heating and the acceleration of solar wind. Several characteristics have
been derived about their velocities, their rates of occurrence, and their
relationship with CMEs. However, the initiation mechanism of jets, e.g.
emerging flux, flux cancellation, or twist, is still debated. In the last
decade coordinated observations of the Interface Region Imaging Spectrograph
(IRIS) with the instruments on board the Solar Dynamic Observatory (SDO) allow
to make a step forward for understanding the trigger of jets and the
relationship between hot jets and cool surges. We observe at the same time the
development of 2D and 3D MHD numerical simulations to interpret the results.
This paper summarizes recent studies of jets showing the loci of magnetic
reconnection in null points or in bald patch regions forming a current sheet.
In the pre-jet phase a twist is frequently detected by the existence of a mini
filament close to the dome of emerging flux. The twist can also be transferred
to the jet from a flux rope in the vicinity of the reconnection by slippage of
the polarities. Bidirectional flows are detected at the reconnection sites. We
show the role of magnetic currents detected in the footprints of flux rope and
quasi-separatrix layers for initiating the jets.
  We select a few studies and show that with the same observations, different
interpretations are possible based on different approaches e.g. non linear
force free field extrapolation or 3D MHD simulation.
