  Spectroscopic observations at extreme and far ultraviolet wavelengths have
revealed systematic upflows in the solar transition region and corona. These
upflows are best seen in the network structures of the quiet Sun and coronal
holes, boundaries of active regions, and dimming regions associated with
coronal mass ejections. They have been intensively studied in the past two
decades because they are highly likely to be closely related to the formation
of the solar wind and heating of the upper solar atmosphere. We present an
overview of the characteristics of these upflows, introduce their possible
formation mechanisms, and discuss their potential roles in the mass and energy
transport in the solar atmosphere. Though past investigations have greatly
improved our understanding of these upflows, they have left us with several
outstanding questions and unresolved issues that should be addressed in the
future. New observations from the Solar Orbiter mission, the Daniel K. Inouye
Solar Telescope and the Parker Solar Probe will likely provide critical
information to advance our understanding of the generation, propagation and
energization of these upflows.
