  One of the many exciting revelations of the New Horizons flyby of Pluto was
the observation of global haze layers at altitudes as high as 200 km in the
visible wavelengths. This haze is produced in the upper atmosphere through
photochemical processes, similar to the processes in Titan's atmosphere. As the
haze particles grow in size and descend to the lower atmosphere, they coagulate
and interact with the gases in the atmosphere through condensation and sticking
processes that serve as temporary and permanent loss processes. New Horizons
observations confirm studies of Titan haze analogs suggesting that
photochemically produced haze particles harden as they grow in size. We outline
in this chapter what is known about the photochemical processes that lead to
haze production and outline feedback processes resulting from the presence of
haze in the atmosphere, connect this to the evolution of Pluto's atmosphere,
and discuss open questions that need to be addressed in future work.
